Acetylcysteine (also known as N-acetylcysteine or N-acetyl-L-cysteine or NAC) is primarily used as a mucolytic agent and in the management of acetaminophen poisoning. Acetylcysteine likely protects the liver by maintaining or restoring the glutathione levels, or by acting as an alternate substrate for conjugation with, and thus detoxification of, the reactive metabolite. Nacystelyn (NAL), a recently-developed lysine salt of N-acetylcysteine (NAC) is known to have excellent mucolytic capabilities and is used to treat cystic fibrosis (CF) lung disease. NAC as a precursor to the antioxidant glutathione modulates glutamatergic, neurotrophic, and inflammatory pathways. The potential applications of NAC to facilitate recovery after traumatic brain injury, cerebral ischemia, and in treatment of cerebrovascular vasospasm after subarachnoid hemorrhage. Acetylcysteine serves as a prodrug to L-cysteine, which is a precursor to the biologic antioxidant, glutathione, and hence administration of acetylcysteine replenishes glutathione stores. L-cysteine also serves as a precursor to cystine, which in turn serves as a substrate for the cystine-glutamate antiporter on astrocytes hence increasing glutamate release into the extracellular space. Acetylcysteine also possesses some anti-inflammatory effects possibly via inhibiting NF-κB through redox activation of the nuclear factor kappa kinases thereby modulating cytokine synthesis. NAC is associated with reduced levels of inflammatory cytokines and acts as a substrate for glutathione synthesis. These actions are believed to converge upon mechanisms promoting cell survival and growth factor synthesis, leading to increased neurite sprouting.

Cysteine (cysteine hydrochloride is a salt) is a thiol-containing amino acid that is oxidized to form cystine. Cysteine is synthesized from methionine via the trans-sulfuration pathway in the adult, but newborn infants lack the enzyme, cystathionase, necessary to effect this conversion. Therefore, cysteine is generally considered to be an essential amino acid in infants.

Dacisteine is a derivative of a N-acetylcysteine, where a second acetyl group is attached to a sulfur atom. Dacisteine is marketed in France under tradename Mucothiol for the treatment of disorders of bronchial secretion, acute bronchitis and acute episode of chronic bronchopneumopathies. Dacisteine exerts its action on the gel phase of the mucus, presumably by breaking the disulfide bonds of the glycoproteins, and thus promotes the expectoration.