L-Cysteine
Binds acetaldehyde directly, and has the only positive randomised trial aimed squarely at hangover symptoms — from nineteen men, with p-values that sit right on the line.
One small positive randomised trial (n = 19) with borderline p-values and a published critique, plus a clean chemical mechanism. Better direct evidence than most of the category and still not enough to expect a reliable effect. Same-night action; nothing to gain the following morning.
Cysteine reacts with acetaldehyde to form 2-methylthiazolidine-4-carboxylic acid (MTCA), a stable, non-toxic adduct that is excreted. That is a real chemical reaction, not a marketing metaphor, and it is why L-cysteine appears alongside DHM in most commercial hangover formulas. The clinical evidence is one small Finnish trial: nineteen men, 1.5 g/kg of alcohol, randomised and placebo-controlled, reporting less morning nausea (p = 0.048) and less stress (p = 0.051) at 1,200 mg, with anxiety benefits appearing at 600 mg. Read those numbers honestly — nineteen people and a p-value of 0.051 is a hypothesis, not a finding, and a published commentary in the same journal made exactly that point. Cysteine is also the rate-limiting precursor for glutathione, which is the more interesting reason to care about it: alcohol depletes hepatic glutathione, and glutathione is what the liver uses to handle both acetaldehyde and paracetamol. If you want to raise cysteine, note that NAC is the better-studied and cheaper route — and that NAC failed to beat placebo for hangover in a larger trial.
Key studies
- L-cysteine containing vitamin supplement and hangover symptoms (Eriksson et al., Alcohol & Alcoholism 2020) — The one positive randomised trial: 19 men, placebo-controlled, 1.5 g/kg alcohol. Nausea p = 0.048 and stress p = 0.051 at 1,200 mg. Genuinely the best direct evidence any hangover ingredient has, which tells you more about the category than about cysteine.
- Commentary on Eriksson et al. (Alcohol & Alcoholism, 2021) — Linked deliberately so you can read the criticism next to the claim. The trial's size and analysis drew a formal published response — the appropriate reaction to a borderline result in nineteen people.
- Form
- Free-form L-cysteine, usually in a slow-release lozenge in the products studied (the Finnish work used a buccal formulation, which keeps cysteine in contact with acetaldehyde in saliva). Plain capsules are not the same delivery.
- Dose
- 600–1,200 mg with alcohol. The nausea and headache signal appeared at 1,200 mg; the anxiety signal at 600 mg.
- Timing
- With the last drinks, or during drinking. It works by intercepting acetaldehyde as it forms — the morning after is too late for the mechanism.
- Schedule
- Occasional. Not a daily supplement.
- Side effects
- Generally well tolerated at these doses. Nausea, sulphurous taste and smell.
- Interactions
- None well characterised at supplement doses. Nitroglycerin and some chemotherapy protocols interact with thiol donors — check if either applies.
- Contraindications
- Cystinuria (cysteine stone formation). Pregnancy and breastfeeding — no data at supplemental doses.
- Lab markers
- None routine.
- Food sources
- Whey, eggs, poultry, garlic, onions, legumes. Dietary cysteine is plentiful; this is about timing a bolus, not correcting a shortfall.
Pairs with
NAC — the same cysteine, better absorbed, far cheaper, and tested in a larger hangover trial where it did not beat placebo. DHM, which it appears alongside in nearly every commercial formula.
within Amino acids & derivatives