NAC (N-Acetylcysteine)

Rescues the liver after a Tylenol overdose

Acetaminophen is mostly cleared safely, but a fraction is converted by CYP2E1 into a reactive metabolite, NAPQI, which glutathione normally neutralizes. In an overdose, hepatic glutathione runs out, NAPQI binds to mitochondrial proteins, and hepatocytes die. NAC supplies cysteine to rebuild glutathione and can also conjugate NAPQI directly. The national multicenter study of oral NAC (1976–1985) showed it prevents severe hepatotoxicity when given within about 8 hours of ingestion, with benefit fading after that.¹ (Strong — decades of clinical use; this is why NAC is a prescription drug before it's a supplement)

This is the cleanest demonstration that NAC does what the supplement marketing claims — raises glutathione in a tissue that needs it. The catch is that it demonstrates this in a depleted liver. A healthy person eating adequate protein has less room to move (see GlyNAC below for who does).

Fewer COPD flare-ups

NAC's free thiol breaks disulfide bonds in mucus glycoproteins, which is its original use as a mucolytic. It also has antioxidant effects in the airway. The two large trials disagree:

• BRONCUS (Europe, 523 patients, 600 mg once daily, 3 years): no effect on lung-function decline or exacerbation rate (1.25 vs 1.29 per year). A subgroup not taking inhaled steroids looked better.²
• PANTHEON (China, 1,006 patients, 600 mg twice daily, 1 year): 22% fewer exacerbations (1.16 vs 1.49 per patient-year).³ Funded by Zambon, the NAC manufacturer, with Zambon staff among the authors.

The most plausible reading is that the dose matters (1,200 mg/day beat 600 mg/day) and that the benefit is concentrated in people not already on inhaled corticosteroids. Treat this as real but modest, and note the funder. (Moderate — two large RCTs, one positive and industry-funded, one null)

Eases compulsions and cravings

This is the most interesting part of the NAC story, and it has nothing to do with antioxidants. Cystine, the oxidized form of cysteine, is traded into glial cells through the cystine–glutamate antiporter (system x_c⁻). For every cystine that comes in, one glutamate goes out. This extracellular glutamate sets a baseline tone on presynaptic mGluR2/3 receptors in the nucleus accumbens, which restrains the bursts of glutamate that drive compulsive, cue-triggered behavior. Chronic drug use and compulsive habits appear to lower that baseline tone. NAC raises cystine, restores the exchange, and so restores the brake. (Animal — well-developed rodent addiction model)

Human trials in this area are small but positive, which is unusual for a supplement:

• Trichotillomania (compulsive hair-pulling), 50 adults, 1,200–2,400 mg/day for 12 weeks: 56% were much or very much improved versus 16% on placebo. The improvement only appeared after about 9 weeks.⁴
• Cannabis dependence in adolescents, 116 participants, 1,200 mg twice daily for 8 weeks alongside contingency management: 2.4 times the odds of a clean urine test.⁵ The authors noted it was the first cannabis pharmacotherapy trial in any age group with a positive primary outcome.

Follow-up trials in other addictions and in adults have been more mixed. (Moderate — small, independent RCTs with a clear mechanism; replication is inconsistent)

Restores glutathione in older adults (with glycine)

Glutathione is made from three amino acids: glutamate, cysteine, and glycine. Cysteine is usually the limiting one, but in older adults glycine also runs short, so NAC alone only fixes half the problem. Sekhar's group at Baylor gave 12 older adults glycine plus NAC ("GlyNAC") for 16 weeks against an alanine placebo. They reported corrected glutathione deficiency, lower oxidative stress, improved mitochondrial fat oxidation, lower insulin resistance, and better gait speed, strength and waist circumference.⁶ (Early — one group, 12 people per arm, very many outcomes reported)

This is the best case for NAC as a general-health supplement: it's targeted at a documented deficiency rather than "more antioxidants is better". It also connects to Glycine / Collagen. The size of the effects, from 24 older adults, needs independent replication before being taken at face value.

How to take it

• Dose: 600 mg once or twice daily covers the lung and general-use trials. The compulsion trials used 1,200–2,400 mg/day, and the effect took 2–3 months to appear.
• Absorption: oral bioavailability of intact NAC is low (single digits to ~10%). Most of it is deacetylated in the gut wall and liver into cysteine, which is the point anyway.
• Pair with glycine: 1 tsp (3–5 g) of glycine powder or a serving of collagen covers the other limiting building block (see GlyNAC).
• Food first: if protein intake is adequate — beef, eggs, fish, dairy — baseline cysteine usually isn't the bottleneck in a young, healthy person. See Cysteine.
• Hangovers: the related amino acid L-cysteine had one small positive hangover trial (see DHM), but there's no good NAC-specific data.
• Regulatory note: NAC is an FDA-approved drug (1963), and in 2020–22 the FDA briefly argued it couldn't also be sold as a supplement. It backed down in 2022 after industry petitions. Availability is fine; the episode is a reminder that this is a drug.

Safety

Training adaptations: exercise-generated reactive oxygen species are a signal for mitochondrial biogenesis and insulin sensitivity, not just damage. High-dose antioxidants taken around training (best shown for vitamins C and E) can blunt those adaptations, and NAC works through the same logic. If training is the goal, don't dose it right before or after workouts. (Inferred for NAC specifically — the direct evidence is on vitamins C/E)

Other: nausea, GI upset and a sulfur smell are the common complaints. IV NAC can cause anaphylactoid reactions, but that's a hospital-dose issue. It may add to the effect of nitroglycerin (hypotension, headache).