L-Cysteine hydrochloride hydrate (L-Cysteine hydrochloride monohydrate) is a conditionally essential amino acid, which acts as a precursor for biologically active molecules such as hydrogen sulphide (H2S), taurine and glutathione. L-Cysteine hydrochloride hydrate suppresses ghrelin and reduces appetite in rodents and humans.

L-Cysteic acid monohydrate (3-Sulfo-L-alanine Monohydrate) is an oxidation product of L-cysteine that may be used as a competitive inhibitor of the bacterial aspartate: alanine antiporter (AspT) exchange of aspartate and in other aspartate biological systems. L-Cysteic acid monohydrate is used in monomeric surfactant development. L-Cysteic acid monohydrate may be used in studies of excitatory amino acids in the brain, such as those that bind to cysteine sulfinic acid receptors. It is a useful agonist at several rat metabotropic glutamate receptors (mGluRs).