DL-Penicillamine is a sulfur-containing amino acid derivative, specifically a dimethylated cysteine. It is a racemic mixture of D- and L-enantiomers. Chemically, it is 3-mercapto-D-valine. Its primary and most significant application is as a disease-modifying antirheumatic drug (DMARD) for the treatment of severe, active rheumatoid arthritis. It acts by reducing inflammation and modulating the immune response, though its precise mechanism is complex and not fully understood. Historically, it was also used as a chelating agent in Wilson's disease (copper overload) and cystinuria, although D-penicillamine is the preferred enantiomer for these indications due to reduced toxicity. DL-Penicillamine is associated with a range of potential adverse effects, including bone marrow suppression, proteinuria, and autoimmune reactions. Its use requires careful patient monitoring. While it is an amino acid derivative, its classification is driven by its established therapeutic role as an active pharmaceutical ingredient.