L-cysteine can be obtained from dietary protein or synthesized from the amino acid L-methionine. It is directly depleted by cigarette smoking, the depletion of which may be partially related to the faster aging of skin in smokers. L-cysteine can act as an antioxidant and may prevent liver diseases.

L-cysteine is a sulfur containing amino acid, which together with glycine and glutamic acid forms the all-important antioxidant glutathione. L-cysteine contributes to the formation of protein in our bodies. It plays a role in energy metabolism, both by being able to convert to glucose and be used as fuel, also by playing a part in the synthesis of fatty acids. It acts in the liver to help detoxify carcinogens and other dangerous chemicals.

Cysteine is essential for T-cell proliferation and activation in your immune system. Cysteine is a major component of skin, hair, and nails. Studies show that cysteine supplementation can increase hair shaft diameter and hair growth density in some cases of hair loss.

Cysteine may be used to:
Help with skin health;
Stop hair loss and aid hair growth;
Aid wound healing from injuries, burns, or surgery;
Help protect us from the adverse effects of alcohol consumption, cigarette smoking, and other chemicals;
Support the liver when there are liver disorders;
Enhance absorption of iron;
Ameliorate chemical sensitivity reactions;
Promote the production of red and white blood cells;
Support cartilage (as a precursor to chondroitin sulfate).