A 2025 rodent study added several reasons to avoid non-emergency use of N-acetylcysteine:

“We previously showed that antioxidants induced an impairment of negative feedback of the hypothalamus-pituitary-adrenal (HPA) axis in rats, in parallel to a down-regulation of glucocorticoid receptor (GR) and nuclear factor erythroid 2-related factor 2 (Nrf2) expression in the pituitary gland. This study evaluated the role of the Nrf2-heme-oxygenase-1 (HO-1) pathway on impairment of negative feedback of the HPA axis induced by N-acetylcysteine (NAC).

Male Swiss-Webster mice were orally supplemented with NAC for 5 consecutive days. The Nrf2-HO-1 pathway activator cobalt protoporphyrin IX (CoPPIX) was injected intraperitoneally on days 2 and 5 after starting NAC supplementation.

NAC reduced expression of Nrf2 in the pituitary of mice. NAC induced adrenal enlargement and hypercorticoidism, along with a decrease in GRα expression and an increase of GRβ expression in the pituitary gland.

We observed that dietary supplementation with NAC ( Figure 4A ) significantly increased plasma corticosterone levels in mice 24h ( Figure 4B ) as well as 15 days ( Figure 4C ) after the last administration of the antioxidant with the same magnitude of response (3.5-fold and 3.4-fold, respectively).

Chronic activation of the HPA axis can have damaging effects on immune, cardiovascular, metabolic, and neural functions, increasing the risk of immune system dysfunction, mood disorders, and metabolic and cardiovascular diseases. To prevent these deleterious effects of chronic hypercortisolism, HPA axis function is controlled by a glucocorticoid-dependent negative feedback system that is essential for ending the stress response.

These findings showed that NAC reduced Nrf2-HO-1 pathway activation in the pituitary gland, in a mechanism probably related to a local downregulation of GRα and an up-regulation of GRβ, leading to a failure of negative feedback of the HPA axis and consequently to the hyperactivity of this neuroendocrine axis.”

https://pmc.ncbi.nlm.nih.gov/articles/PMC11827418/ “Activation of the Nrf2/HO-1 pathway restores N-acetylcysteine-induced impairment of the hypothalamus-pituitary-adrenal axis negative feedback by up-regulating GRα expression and down-regulating GRβ expression into pituitary glands”

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A human equivalent to this study’s NAC dose is (150 mg x .081) x 70 kg = 851 mg. Human supplements are sold in 600 mg and 1000 mg doses.

Grok 3 replied to a question: Human equivalent time to 15 days in male Swiss-Webster mice aged between 4 and 6 weeks? by stating: “15 days in male Swiss-Webster mice aged 4 to 6 weeks corresponds to approximately 4.1 human years, advancing their equivalent human age from about 10–12 years to roughly 14–16 years.” Four+ years seems like a long time for NAC to steadily and continuously affect humans’ HPA axes per the above graphic. What do you think?

Previously mentioned reasons to avoid daily use of NAC were in the lower part of A good activity for bad weather days.