The Impact of N-acetylcysteine on Major Depression: Qualitative Observation and Mixed Methods Analysis of Participant Change during a 12-week Randomised Controlled Trial
Samantha E. Russell1, David R. Skvarc2, Mohammadreza Mohebbi3, David Camfield2, Linda K. Byrne2, Alyna Turner1, Melanie M. Ashton1, Michael Berk1,4,5,6, Seetal Dodd1,5,6, Gin S. Malhi7,8, Sue M. Cotton5,6, Ashley I. Bush4, Olivia M. Dean1,4
1IMPACT−the Institute for Mental and Physical Health and Clinical Translation, School of Medicine, Deakin University, Barwon Health, 2School of Psychology, Faculty of Health, Deakin University, 3Biostatistics Unit, Faculty of Health, Deakin University, Geelong, 4Florey Institute for Neuroscience and Mental Health, University of Melbourne, 5Department of Psychiatry, University of Melbourne, Royal Melbourne Hospital, 6Orygen, The National Centre of Excellence in Youth Mental Health, Parkville, 7Discipline of Psychiatry, Sydney Medical School, The University of Sydney, Sydney, 8CADE Clinic, Department of Psychiatry, Royal North Shore Hospital, St Leonards, Australia
Correspondence to: Olivia M. Dean
IMPACT−the Institute for Mental and Physical Health and Clinical Translation, School of Medicine, Deakin University, Barwon Health, HERB B Level 3, P.O. Box 281, Geelong 3220, Australia
Received: March 15, 2022; Accepted: April 4, 2022; Published online: May 30, 2023.
© The Korean College of Neuropsychopharmacology. All rights reserved.

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Objective: N-acetylcysteine (NAC) is a novel therapeutic agent with multiple mechanisms of action in the central nervous system and a favourable side effect profile. Clinical evidence indicates that adjunctive NAC may reduce the severity of depressive symptoms in individuals with major depressive disorder (MDD).
Methods: A 12-week randomised controlled trial of 2,000 mg/day adjunctive NAC for MDD found no significant improvement at the primary endpoint (week 12) but did see improvements at the post-discontinuation interview (week 16). Within the context of patient-centered treatment, mixed-methods qualitative analysis was also included to explore factors that may determine individual responses to adjunctive NAC treatment. These data were drawn, under blinded conditions, from clinician notes recorded in the case report form. Using the DSM-5 symptom profile for MDD as the initial framework, themes were developed and explored. Frequencies were compared between placebo and NAC groups.
Results: Per protocol analysis of individual themes across the six interviews revealed group differences in favour of NAC for overall depressive affect, optimism, relationships and reduced functional impairment.
Conclusion: This study provides further evidence for the utility of the mixed methods approach complimenting the primary findings using traditional quantitative analyses, as well as being able to capture additional, often more subtle, evidence of individual symptom-level change that reflects improvement in functional abilities in response to NAC supplementation. The use of mixed methods to explore outcomes from psychiatric studies should be considered in future to work towards improved patient-centred care and both confirm quantitative findings and generate novel hypotheses.
Keywords: Depressive disorder, major; Qualitative research; N-acetylcysteine; Psychiatry; Neurosciences

Major depressive disorder (MDD) affects a large proportion of the population, with lifetime prevalence estimated to be 14.4% [1]. It constitutes the third leading cause of burden of disease or injury worldwide [2], with the World Health Organization (WHO) predicting that by 2030 it will be the leading cause of disease burden globally [3]. Current recommended first-line treatments for MDD include antidepressants and psychotherapy [4].

Many people do not report adequate response to standard treatments, with less than 50% of patients responding to the first trialled antidepressant [5]. Further, clinical trials typically report only small effect sizes, with benefits over placebo of around 12% in moderate depression and 16% in severe depression [6]. Antidepressants are also associated with many side effects including weight gain, fatigue, dry mouth, gastrointestinal symptoms, sleep distur-bance and negative impacts on sexual functioning [7-9]. These side effects may compromise treatment adherence [10] with research in primary care settings suggesting that between 43% and 58% of patients discontinue antidepressant treatments within the first 60 days [11,12]. As such, there is an urgent need to develop and evaluate novel therapies for this disorder.

N-acetylcysteine as a Novel Therapeutic

N-acetylcysteine (NAC) is the acetylated molecule of the amino acid L-cysteine. NAC is rapidly absorbed by the stomach and intestines, where a large proportion of it is converted to L-cysteine in the liver [13]. NAC has multiple mechanisms of action and, in turn, wide-ranging effects on brain function. In regards to MDD, many of the pathophysiological processes targeted by NAC are highly relevant to the disorder. MDD is associated with increased levels of oxidative stress, increased inflammatory response, reduced levels of neurotrophic factors and neurogenesis, as well as dysregulation of dopamine and glutamate transmission, all of which are attenuated through the administration of NAC [14].

NAC also has a favourable side effect profile, which may potentially lead to better tolerability and treatment adherence in comparison to conventional antidepressants. A low reported incidence of adverse events occurs with oral NAC doses up to 8,000 mg/day [15]. Minor adverse events that have been commonly reported with NAC include gastrointestinal complaints such as nausea and heartburn, musculoskeletal complaints such as back and joint pain, as well as decreased energy and headaches [16]. However, these side effects are considerably less severe than the those associated with pharmaceutical antidepressants [8].

Clinical Studies of NAC in Major Depressive Disorder

A systematic review and meta-analysis of the use of NAC in depression provides an overview of NAC’s potential utility [16]. Overall the studies (including both bipolar and unipolar depression) found in favour of NAC as an adjunct for depressive symptoms [16].

Our team has investigated NAC for a variety of disorders including MDD [17-20]. In our trial of MDD, 2,000 mg/day NAC (or placebo) was administered over 12-weeks (with a discontinuation interview at week 16) to 252 participants who met the Diagnostic and Statistical Manual of Mental Disorders 4th edition, text revision (DSM-IV-TR) criteria for a current depressive episode. The mean baseline Montgomery-Åsberg Depression Rating Scale (MADRS) [21] score was 27.9 (standard deviation = 5.8), which represents moderate severity of depressive symptoms [17]. Change in depressive symptoms, as mea-sured by the MADRS, did not significantly differ for NAC in comparison to placebo at week 12 but was in favour of the NAC group at week 16 (p = 0.036). The rate of improvement on the Clinical Global Impression [17] of Severity (CGI-S) was greater in the NAC group at the 16-week treatment discontinuation follow-up interview (p < 0.001). Supplementary analyses also revealed lower CGI-S in the NAC group compared with the placebo group across several individual timepoints (weeks 6, 8, 12, and 16) in participants who presented with elevated depressive symptoms at baseline (with MADRS scores > 25). The rate of change of functional improvements was also significantly greater for NAC versus placebo, using endpoint analyses at both 12- and 16- weeks [17,22].

Mixed-methods Data Analysis

One of the challenges facing researchers investigating the psychopharmacological effects of NAC is that it is genuinely a novel psychotropic agent that has potential neurochemical and clinical effects across multiple aspects of brain function [23], and hence multiple clinical pheno-types. For this reason, the therapeutic effects of NAC may not be adequately captured using standardised rating scales that focus on a narrow range of disorder-specific symptoms. In order to investigate the therapeutic effects of NAC more widely, it may be advantageous to include qualitative information obtained both through clinical observation and reports of subjective experience. By combining quantitative and qualitative research methods in the same study, using mixed-methods research (MMR), a more rounded picture of its therapeutic profile may be obtained. While MMR is used commonly in psychological research, this method has so far only been utilised in a limited number of pharmacological RCTs. We were the first to use MMR to study the therapeutic effects of NAC in RCTs [18,24]. In our investigation of adjunctive NAC for schizophrenia, we identified a number of themes relating to mental state improvements that were reported with significantly higher frequency in blinded comparisons of the NAC group com-pared to placebo. These included reports of improved insight, adequate self-care, improved social interaction, improved motivation and volition, improved mood stability and psychomotor stability. Similarly, several themes relating to persistence of symptoms were reported as unchanged or worsened to a greater extent in the placebo group. These included dysthymia, auditory hallucinations, social withdrawal, ideas of reference and grandiosity.

An essential aspect of our additive qualitative analysis was that, not only did it replicate the findings of the primary analysis, but it also revealed new clinical informa-tion regarding patterns of improvement and persistence of symptoms that we had not anticipated [23]. Of particular relevance to the current review, improvements in mood symptoms (dysthymia) associated with NAC consumption were detected in the qualitative analysis that were undetected in the quantitative analysis. Likewise, using the same methodology, in Dean et al. [18], improvements to verbal communication in children with autism spectrum disorder were detected for the NAC group but not control group; a finding that had been suggested in prior case reports but never established in clinical trials (2019). Recently, a similar MMR with bipolar disorder revealed more positive themes associated with NAC consumption compared to control and also identified notable change in participants perceived stresses over the trial [25]. In each of these studies, MMR identified crucial clinical information regarding improvement and persistence of symptoms that was undetected in the quantitative analysis [23]. These hypothesis generating studies not only provide insights for clinical use and practice but also inform future studies about where improvements may be seen and where to focus further quantitative trials.

Study Aims

In the current study, textual data from the Berk et al. [17] 12-week RCT examining the efficacy of NAC in MDD were analysed (under blinded conditions) using the MMR approach. Using a similar approach to the qualitative methods applied by our team previously, two a priori aspects of clinical observation and reporting were examined: (i) participants’ subjective verbal description of the persistence and change in their symptoms over the course of the trial, and (ii) clinician descriptions of their observations and assessment of each participants’ change in mental state across the course of the trial [19,23]. In this way, the potential effects of NAC can be determined according to the full range of somatic, cognitive, affective and functional symptoms, without any prior assumptions in regards to which symptoms would be differentially affected.

The aim of the study was to explore whether, compared to placebo, NAC supplementation would be associated with an altered frequency of themes relating to improvement in symptoms, or themes relating to persistence or worsening of symptoms over the course of the 12-week intervention period and 16-week follow-up visit. We employed methods we have previously used [17,18,25] to explore differences in the sum of themes analysed at each study interview, as well as between-group (NAC compared with placebo) differences in the rate of change of these symptoms over time.


Participants and Study Design

The original trial by Berk et al. [17] was a triple-blind, randomised, placebo-controlled trial, conducted over 12 weeks, and compared the efficacy of adjunctive NAC with placebo in the acute treatment of moderate to severe MDD. In summary, 269 participants were included based on: being aged 18 years or older, fulfilling DSM-IV-TR diagnostic criteria for MDD, single episode or recurrent, and scoring ≥ 18 on the MADRS which is representative of at least moderate depression severity.

Participants were randomly and blindly allocated to receive 2,000 mg/day NAC or placebo, adjunctive to their existing MDD treatments for 12 weeks. Participants attended study interviews on six separate occasions over the 12-week treatment period (baseline, weeks 2, 4, 6, 8, and 12) as well as an additional final follow-up interview at week 16, four weeks after discontinuing treatment. The study was conducted according to Declaration of Helsinki [26] and received appropriate Human Research Ethics Committee approval (Barwon Health HREC 07/24, Royal North Shore Hospital HREC 0708-147M, Bendigo Health Care Group HREC 9/2007, Mental Health Research Institute HREC 07/24_T1). The study was registered on the Australian and New Zealand Clinical Trials Registry: ACTRN12607000134426.

Data Collection

Written qualitative notes of participants’ verbal responses (quotes) and interviewers’ observations of the participants’ affect were recorded in the trial source documents.

At each assessment point, participants were asked: “How have you been since I last saw you?”, “Do you have any specific worries or complaints about your health in general?”, and “Do you have any other problems or worries in relation to the trial?”. Open-ended prompting was allowed to help initiate conversation if needed, for example “Tell me more about that”. Interviewer notes were made both during and immediately after the interviews and included the responses to the initial question as well as any other comments noted by the researcher. Observa-tions of the participant’s affect, appearance, behaviour, and any change or persistence of depressive symptoms was also recorded. Transcripts were not returned to participants for comments or correction. A total of nine researchers conducted the qualitative assessment inter-views. Intensive training and certification processes were undertaken by the interviewers regarding the interview process and they were instructed on the qualitative questions and open-ended prompting.

Qualitative Coding

Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5) [27] criteria for MDD were used as a framework for theme development, although other emerging themes were identified in the data. DSM-5 criteria were selected as they provide more relevant information regarding the current symptom profile of MDD. The original trial was conducted using DSM-IV criteria, but there has been little change to the MDD sections, and the MMR is related to symptom themes rather than diagnosis.

There were 252 participants with post-baseline data available following randomisation, of these 251 had qualitative data available for analysis (125 placebo and 126 NAC). Clinician notes were recorded at each interview, detailing any notable changes in patient symptoms and functioning. All qualitative data was initially transcribed and imported into an excel spreadsheet and the data were imported into NVivo version 11 [28] for qualitative analysis. Textual data were manually coded by SER and DC, who were blind to the treatment allocation until after final data analysis. Coding was conducted to identify themes which were relevant to persistence, worsening or improvement in symptoms related to DSM-5 criteria for MDD (as a starting framework), as well as any other consistent themes (e.g., general functioning or relationship functioning) which emerged from the data. The identified themes were collaboratively refined over time on the basis of regular meetings between SER, DC, LKB, and OMD, in order to arrive at a final list of categories, as displayed in Table 1.

Representative examples of textual reports and how they were coded are also provided in Table 1. Examples of sentences and coding are included here; “Has not done housework as should, frequency and quality both down” (Theme: Functioning impairment), “Poor, anxious, teary, low mood, emotional about recent losses” (Themes: Mood Negative and Anxiety present), “Difficulty sleeping and early morning waking” (Theme: Sleeping issues), “Socially has been isolated in the past week” (Theme: Relationship functioning bad), “Needing to be pushed to do everything” (Theme: Enjoyment/motivation low), “Last week he felt worse than he had ever felt, very depressed” (Theme: Mood negative), “Family issues have deteriorated. Has gone down this visit due to family stress” (Themes: Rela-tionship functioning bad, Mood negative, Anxiety present) and “Reports feeling better than she has in a very long time” (Theme: Mood improved).

Thematic Analysis

An adapted thematic analysis was conducted on the collected qualitative data points at week 2, 4, 12, and 16. This analysis was modified from principles by Miles et al. [29,30] and is detailed elsewhere [25]. The analysis considered some a priori (deductive) themes based on depressive symptoms from the DSM-5 to ensure change of symptoms over time was not overlooked during the analysis. The qualitative data points were uploaded into NVivo and four combing cycles were conducted until theme saturation was reached, then the coding cycles were com-menced. Coding was conducted independently by two investigators (DC and SER). The systematic coding process involved allocating participants quotes into corresponding themes, and/or sub-themes. For example, the participants quote “I feel like I am functioning much better this month” would be coded under overarching theme Func-tioning into sub-theme improved.


During the qualitative analysis all coders were blinded to treatment group allocation to counteract any biases. Analyst triangulation was conducted by SER on 66% of the dataset, specifically, week 2, week 4, week 12, and 16. A total of 18 themes (see Table 1) were identified and analysed: some themes were expanded where appropriate (e.g., functioning was expanded into sub themes; impaired and improved). Codes were checked and reviewed for suitability and correctness by two senior researchers (LKB and OMD) with content expertise.

A high degree of coding reliability was found between DC and SER. Utilizing SPSS version 25 software (IBM Co.) [31] the average Intraclass Correlation Coefficient was 0.813 with a 95% confidence interval (95% CI) from 0.490 to 0.929 (F[18,18] = 6.340, p < 0.001). Similarity of 80% agreement or above between coders is accepted as a sufficient agreement [30]. There-fore, this analyst triangulation supports the rigour of this qualitative analysis and confirmability of the findings by DC and SER.

Frequency Matrix Analysis and Generalised Linear Functions

We used contingency tables to summarise the frequency count of each outcome organised by time point and group.

Poisson generalised estimation equation (GEE) regression models with log link were used to estimate intervention effects. Fixed effect factors for time and group allocation and the time by group interaction were included in the model. GEE technique with a robust unstructured covariance structure was used to account for within-participants repeated measures correlation. Incidence rate ratios and 95% CIs are reported for significant effects. Post-hoc analyses were conducted using chi-square.

Due to a large number of blank responses contained in the textual data, both an intention to treat (ITT) and a per- protocol (PP) approach was used. In the ITT approach, all 251 participants were included in analyses across the six time points, with blank entries assumed to be indicative of ‘no change’ in symptoms. In contrast, for the PP approach, only non-blank textual entries were included in the analyses, with blank entries assumed to be indicative of incomplete case reports. Due to the exploratory nature of the current study, and the over-arching aim of using mixed-methods qualitative analysis to inform hypothesis testing in future quantitative studies, an α = 0.05 significance level was used across all tests, without correction for family-wise error.


Theme Identification

Table 1 displays the 44 deductive and inductive themes (and sub-themes) that were identified during the NVivo coding process. There was 2 deductive themes (Depressive affect and Functioning) and 15 inductive themes that arose from the data combing process (Anxiety, Anger/frustration, Employment, Energy, Outlook, Other psychiatric symptoms, Life events, Medications, Mood, Physical health, Suicidality, Sleeping issues, Substance use, Enjoyment/ motivation and Relationship functioning). Table 1 also illustrates exemplar responses coded into the themes.

Two major themes and two minor themes were identified. The major themes were Depressive Affect and Functioning and the minor themes were Optimism and Pessimism. The major theme of Depressive Affect is a deductive a-priori theme as utilised from the DSM-5 diagnostic criteria for MDD. The Depressive Affect theme was then categorized into 6 sub-themes (Improved, Worsened, Mixed, Positive, Negative, Unchanged) as per the codes emerging from the qualitative data. Similarly, the major theme Functioning was categorized into sub-themes of Impaired, Good and Improved. The minor sub-themes Optimism and Pessimism were inductively derived through the data combing process.

Mixed Methods Treatment Effects for Individual Themes

Charts displaying change over time in theme frequencies from the per protocol analysis are presented in Figure 1. The summary of the frequency of individual reported themes are outlined in Table and intervention effect are displayed in Table 2.

Depressive affect improved: We observed a statistically significant linear of effect of time where we observed increased levels of “depressive affect improved” in both groups over time.

Functioning good (participant responses): We observed no significant overall main effects or interactions. However, in the post-hoc simple effects (Supplementary Table 1; available online) we observed large variability in reported outcomes where both groups substantially increased in levels of “functioning good” responses from week 2 to week 4 before sharply dropping at week 12.

Functioning improved: We observed a significant effect of time whereby both groups decreased in levels of reports of improved functioning, as is evident in the significant drop in report frequencies for this outcome from week 4 to week 12 across the sample (illustrated in Fig. 1). In the post-hoc simple effects (Supplementary Table 1; available online) we observed a significant between groups difference, favouring NAC, at week 4. This is perhaps indicative of a good, immediate effect on function that does not persist over time.

Relationship bad (participants response): We observed no overall effects for reports of “relationship bad” for the entire sample, and our post-hoc analysis of the simple effects suggests that the placebo group demonstrated little within-group variability (see Fig. 1, Supplementary Table 1; available online). The NAC group reported a significant decline in “relationship bad” from week 2 to week 4, which also coincides with a significant between-groups difference at T2 with the NAC group reporting significantly fewer counts of “relationship bad”. By week 12, the two groups stabilised and were no longer significantly distinct from each other at any time point, nor from their original scores at week 2. As such, whatever effect might exist for this variable appears to be short-lived.

Optimism (participant responses): We observed a significant interaction between time and group when examining Optimism (incidence rate ratio [IRR] = 1.33; 95% CI: 1.09−1.62, see Fig. 1). The post-baseline rate in optimism increased by 33% in the intervention compared with control. This is likely driven by the significant between groups difference in reported optimism at week 2 (favouring placebo) that was partially reversed at week 4. The NAC group recorded significantly greater levels of optimism at week 4 compared to week 2, while the opposite was true for the placebo group (see Table 2).

Depressive affect positive: We observed a statistically significant interaction between time and group membership (IRR = 1.40; 95% CI: 1.02−1.93, see Fig. 1, Table 2). We observed increased levels of “depressive affect positive” in both groups over time, but the increase was greater in the NAC group, reaching statistical significance by week 16 (see Supplementary Table 1; available online).


In this blinded analysis of the between-group differences according to individual themes at the week 16 follow-up interview, clinician notes indicated an increased frequency of reported improved depressive symptoms, improved affect and increased optimism in the NAC group, as well as an increased frequency of reported depressive symptoms and negative mood in the placebo group. These findings are again consistent with the findings of [17] for the clinical impression of change at the 16-week follow-up and suggest that the improvements in depressive symptoms which resulted from 12-weeks of NAC intervention, compared to placebo, were still maintained up to a month after the conclusion of the treatment period.

Regarding specific themes, the significant positive themes related to improved or positive comments about depressive affect, improved or good functioning, and increased optimism. An example can be seen within Table 1 of the exemplar responses, (Theme: Depressive affect improved) “Participant looks much brighter, smiled, cracked a few jokes, laughed at herself, indicated mental relationship improved”, “Looking forward to holidays & work. Look brighter in affect”, and another report on (Theme: Functioning improved) states “Functioning better & doing things that had not done before” and “Manages well at work & is better organised”. Reports of increased optimism were widely spread within the qualitative codes with examples such as “Coping well, busy time but coping”, “Enjoying people more”, and “Mood has been up & down but has felt more optimistic in the past few days”. These results are consistent with those of Russell et al. [25] who also found significantly more positive themes (including interpersonal relationships, optimism, improved functioning and mood) within the NAC treatment group in MMR for bipolar depression.

Negative themes that emerged as being significantly greater in the placebo group were related to relationships. This is reflected in the qualitive data, an example of exemplar reports “Not getting on well with her current partner and feels dissatisfied by their relationship. Also has a strained relationship with her father and sister which has caused a lot of upset” and “Relationship with wife and kids has been difficult”. Curiously, there is a signal from the NAC autism literature regarding a reduction in irritability which suggests a causal path [32].

This mixed methods qualitative study discovered a number of unexpected findings that were not elucidated in the quantitative analysis. A number of themes that did not reach statistical significance in the quantitative analysis are still notable. First, the commonly mentioned of minor theme of “Outlook” and its sub-themes optimism and pessimism. Reports of optimism, hope and confidence were common amongst the qualitative reports from participants, this may not have been accurately captured by the validated scale measures of the original quantitative analysis by [17] which excludes the opportunities for participants to comment freely about their current outlook. The importance of these themes can be seen by the frequent participant mentions and discussion. Examples can be seen in exemplars “Feels more positive than he has in a long time” and “Had a few interpersonal difficulties which she felt she dealt with well”. Furthermore, the major theme “Outlook”, and its sub-themes optimism and pessimism are the highest coded themes within the qualitative analysis with 651 total codes. Second, the theme of Physical Health, sub-theme Illness had increased mentions in the qualitative data. This theme included common mentions of headaches and also gastrointenstinal symptoms, supporting the original quantitative analysis by Berk et al. [17] which found increased gastrointestinal adverse events in the NAC group, as seen in the exemplars (Theme: Physical Health, sub-theme: Illness) “Has had a few headaches”“Has had nausea which was really bad within 30 mins of taking capsules and also had headaches” and “Continues to report some indigestion (mild) and constipation at times”.

In assessing the utility of mixed-methods analysis, it is noteworthy that the current study replicated the main finding of the original quantitative analysis by [17]. Together, these findings provide further evidence to suggest that the mixed methods approach augments traditional clinical trials, and can capture phenomena that may not be collected with scale measures. Importantly, qualitative data capture includes and supports participants in their treatment development and decision making processes. Mixed methods adds to support the results of quantitative clinical trials, and is hypothesis generating in that it allows for further research themes to emerge from the participant experience.

In the current study, the differences found in favour of NAC for individual themes included depressive affect, relationships, optimism and functioning. Of note, a pattern of increased functional impairment together with related symptoms of reduced energy and motivation was observed in the placebo group, relative to the NAC group, from week 4 onwards. A hypothesis that has emerged from this study, and which may be tested in future studies, is that NAC has some of its most robust effects on functional abilities for individuals with MDD.

There are a number of limitations associated with the current study. There were a large number of entries which were either left blank or contained a very brief textual description, e.g., one-word responses such as ‘good’ or ‘OK’ in response to ‘how have you been since I last saw you?’. In these cases, it was difficult to confidently determine whether these entries were associated with symptom change. The cases with short positive answers were typically coded under one of the more general terms such as ‘mood improved’ or ‘depressive affect improved’, and for this reason, the frequency of reports in these categories is higher than for the other more specific symptom descrip-tions. In the case of blank entries, these were considered as either evidence of no change or missing data in the ITT and PP analyses, respectively. It is also possible that the differences in the number of textual responses, particularly regarding clinician notes, may have resulted from individual variation in clinician approaches across the different participants. While trial clinicians were instructed to be comprehensive in their note taking, there were variations in the data.

A methodological limitation of the study was that no interviewer or participant data were available at baseline (only demographic details and history of the presenting problem). For this reason, it was not possible to determine the rate of change in symptom reports over the first two weeks of the study. In future studies, the inclusion of qualitative data at baseline would address this issue. Finally, it is important to acknowledge that due to a large number of non-parametric tests conducted in the current study, there is likely to be an inflated type-I error rate. However, it is noteworthy that in the Poisson Loglinear regression analysis, where we were able to control the error rate to a much greater extent, a similar pattern emerged namely increased reporting of positive symptoms in the NAC group compared to placebo.

In summary, the current study used blinded textual qualitative data from clinician notes and participant reports to identify themes associated with persistence or improvement in symptoms associated with a 12-week NAC intervention study in individuals with MDD. Analysis of individual themes across the trial interviews revealed group differences in favour of NAC for overall depressive symptoms, mood, affect, pessimism, motivation and enjoyment, functional impairment, and suicidal ideation and self-harm. This study provides further evidence for the utility of the mixed methods approach in being able to complement the primary findings from traditional quantitative analyses, as well being able to capture additional evidence of individual symptom-level change such as improvement in functional abilities with NAC. It supports mixed methods analyses as a potentially valuable adjunct to traditional methods, especially in the study of novel agents with novel modes of action.


The authors would like to thank the participants of the study.


SER is supported by an Australian Government Research Training Program Scholarship. DS has NHMRC support from the Medical Research Future Fund (1200214). DC conducted thematic analysis of qualitative data for this study as part of the course requirements for a Masters of Clinical Psychology at Deakin University, with no other funding sources to declare. AT has received travel/grant support from NHMRC, AMP Foundation, Stroke Foundation, Hunter Medical Research Institute, Helen Macpherson Smith Trust, Schizophrenia Fellowship NSW, SMHR, ISAD, the University of Newcastle and Deakin University. MMA has received grant/research support from Deakin University, Australasian Society for Bipolar Depressive Disorders, Lundbeck, Australian Rotary Health, Ian Parker Bipolar Research Fund and Cooperative Research Centre for Mental Health and PDG Geoff and Betty Betts Award from Rotary Club of Geelong. LKB has received support from the Meat and Livestock Board, Australia-China Science and Research Fund; Workwell Mental Health Improve-ment Fund; The Australian 5G Innovation Initiative, and Deakin University. MB has received Grant/Research Support from the NIH, Cooperative Research Centre, Simons Autism Foundation, Cancer Council of Victoria, Stanley Medical Research Foundation, Medical Benefits Fund, National Health and Medical Research Council, Medical Research Futures Fund, Beyond Blue, Rotary Health, A2 milk company, Meat and Livestock Board, Woolworths, Avant and the Harry Windsor Foundation, has been a speaker for Abbot, Astra Zeneca, Janssen and Janssen, Lundbeck and Merck and served as a consultant to Allergan, Astra Zeneca, Bioadvantex, Bionomics, Collaborative Medicinal Development, Janssen and Janssen, Lundbeck Merck, Pfizer and Servier − all unrelated to this work. AIB receives support from the National Health and Medical Research Council (L3 Leadership Fellow 1194028). He is a shareholder in Alterity Ltd, Cogstate Ltd and Mesoblast Ltd. He is a paid consultant for, and has a profit share interest in, Collaborative Medicinal Development Pty Ltd. He has received lecture fees from Biogen Ltd. OMD is an R.D. Wright Biomedical NHMRC Career Development Fellow (1145634) and has received grant support from the Brain and Behavior Foundation, Marion and EH Flack Trust, Simons Autism Foundation, Australian Rotary Health, Stanley Medical Research Institute, Deakin University, Brazilian Society Mobility Program, Lilly, NHMRC, Australasian Society for Bipolar and Depressive Disorders and Sevier. She has also received in-kind support from BioMedica Nutracuticals, NutritionCare and Bioceuticals.


OMD is an R.D. Wright Biomedical NHMRC Career Development Fellow (1145634). The overarching clinical trial was funded by the National Health and Medial Research Council and Australian Rotary Health. MB is supported by a NHMRC Senior Principal Research Fellowship (1156072).

Author Contributions

Original trial investigator: Olivia M. Dean, Michael Berk, Seetal Dodd, Gin S. Malhi, Ashley I. Bush. Sub- study planning and project conception: Olivia M. Dean, Michael Berk, Linda K. Byrne, David Camfield. Data coding/thematic analysis: Olivia M. Dean, Linda K. Byrne, Samantha E. Russell, David Camfield. Data analysis: Mohammadreza Mohebbi, David R. Skvarc, Samantha E. Russell. Figure/table preparation: Olivia M. Dean, David R. Skvarc, Samantha E. Russell, Mohammadreza Mohebbi. Manuscript preparation: Olivia M. Dean, Michael Berk, Melanie M. Ashton, Mohammadreza Mohebbi, Alyna Turner, Linda K. Byrne, David R. Skvarc, Seetal Dodd, Sue M. Cotton, Samantha E. Russell, Ashley I. Bush, Gin S. Malhi, David Camfield.

Fig. 1. Incidence rate ratio (IRR) and 95% confidence intervals (95% CIs) from the per protocol analysis. Presented are the main effects for time and group allocation and the time by group interaction were tested for each outcome using log-linked, generalised linear models for Poisson distributed dichotomous outcomes. The statistically significant themes are displayed; depressive affect improved (A), depressive affect positive (B), functioning good (C), optimism (D), relationship bad (E), functioning improved (F), over the course of the 12 weeks of treatment and 4 weeks after discontinuation. NAC, N-acetylcysteine.

Deductive and inductive themes and sub-themes and examples of responses in each theme

Theme Sub theme Placebo (total frequency) NAC (total frequency) Example response
Major themes
Depressive affect Improved* 77 83 “Participant looks much brighter, smiled, cracked a few jokes, laughed at herself, indicated mental relationship improved”
“Looking forward to holidays & work. Look brighter in affect”
Worsened 58 46 “Seemed a little worse today, she displayed very little affect”
Mixed 28 25 “Not too bad, still depressed but has a couple of good days”
Positive* 78 99 “Feels good, has been stable over the past 2 weeks”
Negative 142 126 “Described being flat last week with a feeling of nothingness”
Unchanged 25 14 “Appeared very much unchanged, displayed some affect but still on the flat side”
Functioning Impaired 82 67 “Finds it hard to sustain concentration on studying and staying awake to do so. Is not able to complete the work quickly”
“Manages well at work & is better organised”
Good* 51 52 “Able to function and attend work”
“Has been able to go to the gym twice a week”
Improved* 124 119 “Functioning better & doing things that had not done before”
Minor themes
Outlook Optimism* 187 189 “Mood has been up & down but has felt more optimistic in the past few days”
“Feels more positive than he has in a long time”.
Pessimism 168 107 “Very flat and low mood, doesn’t see things getting better from himself”
Other themes
Cognitive functioning Difficulties 21 30 Still having concentration lapses”
Improved 21 22 “Thinking a little clearer”
Anxiety Improved 42 51 “A bit better, less anxious in the mornings”
Present 192 236 “Work is stressful, she feels under a lot of pressure”
Anger/frustration Present 63 60 “Some angry days, not wanting to talk to anybody.”
“Feels she has no direction and feels stressed irritated and frustrated”
Employment Started 20 32 “Has started a job after period of unemployment”
Stopped 2 1 “Quit job”
Other psychiatric symptoms NA 45 41 “Had another bulimic episode”
Life events Positive 67 50 “Looking forward to becoming a grandma”
Negative 22 26 “Has been quite upset due to car accident”
Medications Decrease 9 10 “Coming off Oxycontin, now on 5mg”
Increase 10 4 “Has increased Valium to help her relax as she’s been feeling a lot more anxious”
Start 14 19 “Feels more cognitively impaired since starting Lithium”
Stop 11 7 “Changing anti-depressant and phasing out SSRI”
Mood Fluctuating 81 75 “Mentally been fluctuating with some fair days and some ordinary days”
Improved 113 152 “Feels that mood has improved slightly”
Negative 308 243 “Mood still low”
Positive 232 286 “Good, mood started picking up a week ago”
Unchanged 107 93 “No change, doesn’t feel any different”
Worsened 105 102 “Very flat and low mood, doesn’t see things getting better for himself”
Physical health Illness 58 69 “Has had nausea which was really bad within 30 minutes of taking capsules and also had headaches.”
Energy Increased 22 30 “Feels she has more energy”
Decreased 30 22 “Very lethargic, hard to get up in the morning, more tired than usual”
Suicidality Ideation 21 21 “Describes having fairly constant suicidal thoughts but no intention to act on them”
Decreased 2 2 “Not as many suicidal thoughts”
Sleeping issues Improved 61 69 “Sleeping better, feeling more rested”
Present 72 69 “Very poor sleep”
Worsened 29 22 “Sleep has been worse, troubler sleeping and waking a lot”
Substance use Decreased 9 7 “Has reduced drinking in an attempt to improve mood”
Increased 22 30 Was feeling very low on weekend and had a bottle of wine and 10 cigarettes in one evening”
Enjoyment/motivation Good 5 8 “Increased concentration and motivation, enjoying gardening and being outside”
Low 158 166 “Does not enjoy anything at the moment”
Relationship functioning Bad* 118 133 “Not getting on well with her current partner and feels dissatisfied by their relationship. Also has a strained relationship with her father and sister which has caused a lot of upset”
Good 53 38 “Socialised well with family in the last week”

NAC, N-acetylcysteine; NA, not applicable.

*pvalue < 0.05.

Between group relative difference in significant theme frequencies measured by incidence risk ratio in NAC and placebo group at post-baseline follow-up estimated by two-way interaction between group and time using intention to treat analyses

Theme Week 2 Week 4 Week 12 Week 16 Total frequency IRR Lower
CI 95%
CI 95%
z pvalue

Placebo NAC Placebo NAC Placebo NAC Placebo NAC Placebo NAC
Depressive affect improved 17 6 14 21 17 17 29 39 77 83 1.33 0.99 1.78 1.89 0.059
Depressive affect positive 9 2 8 12 27 30 34 55 78 99 1.40 1.02 1.93 2.06 0.041
Functioning improved 47 26 59 77 5 3 13 13 124 119 1.16 0.91 1.48 1.18 0.240
Optimism 83 55 68 81 18 23 18 30 187 189 1.33 1.09 1.62 2.82 0.005
Relationship bad 35 44 32 17 23 35 28 37 118 133 1.10 0.88 1.37 0.82 0.412
Functioning good 22 16 6 20 5 1 18 15 51 52 0.92 0.65 1.31 −0.45 0.655

NAC, N-acetylcysteine; IRR, incidence risk ratio; CI, confidence interval.

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