NAC has a variety of impact on the microbiome, over 800 bacteria impacted. Predicted impact of NAC on the microbiome reported for Autism is that it will increase the shifts overall.
This post looks at reported studies (often these studies are very small samples which makes results unreliable).
- The Use of N-acetylcysteine Supplementation to Decrease Irritability in Four Youths With Autism Spectrum Disorders  Positive subjective response, but ” the authors cannot know whether use of NAC or other medication or behavioral strategies were responsible for such changes because this study was not a controlled trial.”
- Acetylcysteine for Treatment of Autism Spectrum Disorder Symptoms  Subjective improvement in one person “small controlled trials and case reports suggest that acetylcysteine use is associated with improvements in irritability and aggression in prepubertal children with ASD; “
- A Randomized Double Blind Placebo Controlled Clinical Trial of N-Acetylcysteine Added to Risperidone for Treating Autistic Disorders  “The mean score of irritability in the NAC+risperidone and placebo+risperidone groups at baseline was 13.2(5.3) and 16.7(7.8), respectively. The scores after 8 weeks were 9.7(4.1) and 15.1(7.8), respectively. …. The most common adverse effects in the NAC+risperidone group were constipation (16.1%), increased appetite (16.1%), fatigue (12.9%), nervousness (12.9%), and daytime drowsiness (12.9%). There was no fatal adverse effect.”
There are subjective reports of improvement that looked only at irritability and did not report on changes of other symptoms. The studies tend to focus on high irritability ASD.
Dosage from studies: NAC (1200 mg/day). NAC has a half life of 6 hrs, so a dosage of 300 mg four times a day, or 400mg three time a day should be considered.