← The FormularyLong gameL-cysteine hydrochloride
Also known as L-cysteine HCl monohydrate. Cysteine.The right molecule. Not the studied amount.
This is the ingredient where the chemistry is right and the arithmetic is not. Cysteine really is the bottleneck for glutathione, with human isotope studies behind it, and that is why it turns up on liver blends. Then you look at the numbers. The 2011 study that raised glutathione in older adults used cysteine at 0.81 mmol per kilogram per day, which for an 80 kg man is on the order of 8 grams of cysteine daily, and it was delivered as N-acetylcysteine rather than as free cysteine. The recognised sulphur amino acid requirement is about 15 mg per kilogram per day, so roughly a gram for a 70 kg man, and average diets supply more than twice that. We include 10 mg of the hydrochloride salt, which is 6.9 mg of cysteine: under one per cent of a day's requirement, and a fraction of a per cent of what the trials gave. If you want the glutathione-precursor effect the research describes, you want NAC at grams from a bottle that says NAC on it. What our 10 mg does is fill an amino acid position in a botanical blend. It is not doing the thing its reputation is built on, and we are not going to pretend it is.
A sulphur-containing amino acid, supplied as its hydrochloride monohydrate salt because free cysteine oxidises to cystine on contact with air. Ten milligrams of the salt works out at about 6.9 mg of cysteine itself.
Glutathione is a tripeptide of glutamate, cysteine and glycine. The enzyme that joins the first two, glutamate-cysteine ligase, is the rate-limiting step, and cysteine is the rate-limiting ingredient: withhold dietary sulphur amino acids from healthy adults and whole-blood glutathione synthesis slows. That is the entire argument for a cysteine supplement. The limits are the interesting part. The body already makes cysteine from methionine by the transsulfuration pathway, ordinary food supplies grams of sulphur amino acids daily, and free cysteine is a poor delivery vehicle next to N-acetylcysteine, which resists oxidation, dissolves better and is what almost every trial actually used.
Cysteine is one of the three building blocks the body uses to make glutathione, its own antioxidant molecule. It is studied as the supply-limiting input to that process.
Anyone who wants the amino acid position covered in a daily blend and understands it is a token amount. Not for anyone with cystinuria or a history of cystine stones. Not for anyone chasing the glutathione effect described in the literature, which means NAC at gram doses and belongs in a different product entirely. Not for anyone treating a capsule as insurance against how they drank last night.
10 mg daily with food, as part of the LIVER blend. Nothing to time and nothing to avoid. If you already take NAC separately, this adds nothing meaningful on top and does not need to be spaced apart from it.
Nothing you will notice, and nothing a blood test would register at this amount. Ten milligrams of cysteine is not a felt ingredient. It is a listed one.
That cysteine supply limits glutathione synthesis is established in humans, though on small numbers. Lyons and colleagues, PNAS 2000 (97:5071-76), fed seven healthy men an adequate diet and a sulphur-amino-acid-free diet for ten days each; whole-blood glutathione fractional synthesis rate fell from 0.65 to 0.49 per day and the absolute synthesis rate from 747 to 579 micromol per litre per day. Sekhar and colleagues, American Journal of Clinical Nutrition 2011 (94:847-53), restudied eight older adults after 14 days of supplementation with cysteine at 0.81 mmol/kg/day, given as N-acetylcysteine, plus glycine at 1.33 mmol/kg/day, and reported glutathione concentrations 94.6 per cent higher, fractional synthesis rate 78.8 per cent higher and absolute synthesis rate 230.9 per cent higher, with lower plasma F2-isoprostanes. That study had no control arm. The pharmacology literature is consistent that NAC is more soluble and more resistant to oxidation than free cysteine; Whillier and colleagues, Redox Report 2009, found that extracellular cystine sustained glutathione synthesis in human red blood cells in vitro at only about 20 per cent of the maximum rate reached with cysteine or NAC. A 2024 systematic review and meta-analysis in the Journal of Cellular and Molecular Medicine pooled 20 NAC supplementation studies of exercise biomarkers. EFSA has separately assessed L-cysteine and L-cysteine hydrochloride as flavouring feed additives and considered them safe for target species at up to 25 mg/kg complete feed, which is an animal feed assessment and not a human efficacy finding. Not supported: any claim that 10 mg of L-cysteine hydrochloride raises glutathione, protects the liver or detoxifies anything.
Read the labelAvoid if you have cystinuria or a history of cystine kidney stones. Free cysteine is less well tolerated than N-acetylcysteine and is not a form to push to high doses; stomach upset is the usual complaint with thiol amino acids. Not a detox, and not a treatment for any liver condition.
Evidence grades describe how well an ingredient is studied — A well-supported, C emerging — not a promise of results. Educational, not medical advice. Doses are typical ranges; follow the label on the product.