L-Cysteine
Also known as: free-form L-cysteine.
Two real but bounded roles: rate-limiting amino acid for glutathione synthesis, and a local thiol trap in saliva, gastric lumen, and portal flux. At swallowed-capsule doses the bulk-plasma trap claim is stoichiometrically implausible — ALDH2 clears plasma acetaldehyde, with NAD⁺ as the rate-limiter. Tier 2 because NAC is the more efficient GSH precursor; L-cysteine is retained as a parallel sulfur substrate and for the local-compartment trap activity NAC cannot match.
Where this fits in the system
These are GSH-substrate doses. Trap-mechanism dosing for local compartments (mouth, stomach) is outside this page's scope — it requires a slow-release buccal or gastric format, not a swallowed capsule.
What it does
L-cysteine has two distinct roles. First, it's the rate-limiting amino acid for glutathione synthesis, which the liver burns through during ethanol clearance. Second, the thiol group can react directly with acetaldehyde to form a thiazolidine adduct — but at the doses achievable from a swallowed capsule, this trap reaction is meaningful only in local compartments where cysteine is concentrated (saliva, gastric lumen, hepatic portal flux). Bulk plasma acetaldehyde is cleared by ALDH2, not by trapping. The Eriksson 2020 trial outcomes are most consistent with the GSH-substrate mechanism, not bulk neutralization.
How it works
The two roles
Ethanol metabolism runs through alcohol dehydrogenase (ADH) to produce acetaldehyde, then through aldehyde dehydrogenase (ALDH2) to clear it. ALDH2 is the dominant clearance pathway in non-ALDH2*2 livers and its rate-limiter is NAD⁺, not enzyme abundance. The classic "L-cysteine traps acetaldehyde" framing misses this: stoichiometric trapping at supplement doses is not what's actually happening to bulk plasma acetaldehyde.
What L-cysteine does at swallowed-capsule doses is real but narrower. It is the rate-limiting amino acid for glutathione synthesis, the liver's primary defense for the catalytic Cys302 residue on ALDH2 itself. And it does form a thiazolidine adduct with acetaldehyde — but only in the compartments where cysteine actually accumulates: saliva, gastric lumen, hepatic portal flux. In bulk plasma the cysteine concentration is too low for the trap to matter.
Buying guidance
Free-form L-cysteine in 500 mg capsules. If your goal is glutathione support and you only buy one thing, NAC does it more efficiently. Free-form L-cysteine is included as a parallel sulfur substrate alongside NAC and for any portal/gastric trap activity. For meaningful local trapping in mouth or stomach (relevant only in upper-GI cancer-risk reduction, especially for PPI users, atrophic gastritis, or H. pylori carriers), only slow-release buccal lozenges work — and that format is not currently sold in the US market.
Deep science · L-Cysteine — deep dive
Why dose matters more here than anywhere else
A standard drink produces roughly 0.3 mol acetaldehyde at peak ADH flux. Stoichiometric coverage would require ~37 g of cysteine per drink — three to four orders of magnitude beyond consumer dosing. The dose-response in trials therefore reflects glutathione throughput and local trapping, not bulk plasma scavenging. For systemic acetaldehyde clearance, ALDH2 is the dominant pathway and NAD⁺ availability is the rate-limiter; that's the lever NR moves.
How it complements NAC
NAC is the more efficient GSH precursor (better bioavailability, no thiol-taste limit on dose). L-cysteine is included as a parallel sulfur-pool substrate and for any local trap activity through portal and gastric compartments. The two are not interchangeable but the NAC-first framing is correct.
Thiazolidine formation kinetics
The reaction between L-cysteine and acetaldehyde proceeds at physiological pH without catalysis. Rate scales with cysteine concentration — which is why the trap mechanism only matters in compartments where cysteine concentrates (saliva, gastric lumen, portal flux), not in bulk plasma where dietary cysteine sits at micromolar levels. Product is 2-methylthiazolidine-4-carboxylic acid, excreted renally.
The Eriksson 2020 trial
A placebo-controlled human trial (Eriksson et al., Alcohol and Alcoholism 2020) tested an L-cysteine-containing vitamin supplement against placebo in healthy volunteers after acute ethanol exposure. Symptom scores — nausea, headache, stress, anxiety — were significantly reduced at the higher cysteine dose. The trial result is real; the most defensible interpretation is glutathione-substrate effect plus local-compartment trapping, not bulk plasma neutralization. The doses used (600 mg and 1200 mg) are stoichiometrically incapable of trapping the bulk acetaldehyde produced by the ethanol exposure in the protocol.
Format matters
Slow-release buccal lozenges (Salaspuro 2002, 2006; Acetium product, EU only) achieve >90% reduction in salivary acetaldehyde at single-digit milligram doses, because the cysteine sits where the acetaldehyde is. Standard swallowed capsules absorb in the duodenum and bypass the oral cavity and most of the gastric compartment. If the goal is local trapping in mouth or stomach (relevant for upper-GI cancer-risk reduction, especially in PPI users, atrophic gastritis, or H. pylori carriers), only slow-release buccal/gastric forms work. For systemic GSH support, the swallowed capsule is fine — but at that point NAC is the more efficient choice.
Layer interaction
L-cysteine feeds the glutathione pool the liver uses to protect ALDH2's catalytic Cys302 from auto-inactivation by acetaldehyde itself. That defensive role is the GSH-substrate mechanism, and it's the role this page's doses are sized for. The bulk-trap framing the protocol used to lead with overstated what a swallowed capsule can deliver; this page now reflects the corrected mechanism.