L-Cysteine

Cysteine · L-Cystine · Cystine

Snapshot

Indications	Contraindications
Hangover symptoms¹ Chronic telogen effluvium² Androgenetic alopecia² Skin pigmentation^3–5 Postoperative inflammation^{6, 7} Oxaliplatin neuropathy⁸ Transient infantile liver failure⁹	Pregnancy or lactation¹⁰ Children¹⁰

L-cysteine is a conditionally essential sulfur amino acid and the rate-limiting building block for glutathione, the body's main intracellular antioxidant¹¹. Supplementation aims to raise glutathione availability.^{3, 7} The uses that follow include recovery after surgery and skin pigmentation^{3, 7}. Cysteine demand rises in stressed and inflammatory states, so the typical patient is one with higher physiologic need, including critically ill patients on nutrition support¹¹.

Most trials test cystine inside combination products, so the effect of cysteine alone is hard to isolate. A small randomized trial pairing cystine with theanine found lower inflammatory markers after esophageal cancer surgery⁷. A randomized trial of a cystine-based supplement with botanicals reported higher hair density in chronic telogen effluvium and androgenetic alopecia. High-dose intravenous cysteine did not raise glutathione synthesis in preterm infants. In a 2020 randomized, placebo-controlled study in men given alcohol, L-cysteine eased hangover nausea and headache on correlational analysis, in a small sample. Products listing L-cysteine are mostly capsules, alongside N-acetylcysteine products; see Form comparison.

Some United States product labels state the product is not intended for pregnant or lactating women¹⁰. Health Canada labels ask for a practitioner's advice before use in pregnancy or breastfeeding¹².

Last updated October 17, 2026. Read how we make monographs.

In the full monograph

46 sources, 35 studies

When to useThe situations practitioners use it for, each with the reason and the study behind it.
SafetyWho should avoid it, who needs extra care, and the side effects reported in studies.
InteractionsThe medicines and supplements it interacts with, the effect of each, and what labeling says to watch.
Protocol notesThe doses used in studies, how and when they were taken, and how the forms on the market compare.
How it worksWhat it is and how it works in the body, in plain terms.
Published studiesThe studies behind each use, popular claims checked against them, and trials still under way.
Word of mouthWhat patients and practitioners report online, kept separate from the clinical evidence.
Clinical perspectiveA closing read of where the evidence is solid, where it is thin, and what has not been studied.

Snapshot

In the full monograph

Sources