a group of antimicrobial drugs derived from sulfanilic acid. Their antibacterial properties were discovered by the German pharmacologist G. Domagk in 1934–35.

Sulfa drugs are similar in chemical structure to para- aminobenzoic acid (PABA), an essential factor in the growth of microorganisms; its absence prevents microbial reproduction. The drugs function by competing with PABA for binding with certain enzymes in the microbial cell. When the drugs combine with enzymes, bacteria lose their ability to synthesize folic acid, a vitamin essential to them. The bacteria then lose the ability to transform the substances that they normally transform together with PABA. Since the enzymes have a greater affinity for PABA than for the sulfa drugs, a therapeutic effect is achieved by fairly large doses of the drugs. Insufficient dosage or the premature cessation of therapy may give rise to strains of the causative agents that are not affected by sulfa drugs. The bacteriostatic effect of sulfa drugs extends to many microorganisms, including streptococci, pneumococci, staphylococci, gonococci, meningococci, and some large viruses.

Sulfa drugs that remain in the body a short time include strep-tocide, sulfaethidole, and sulfamethazine; long-acting sulfa drugs include sulfapyridine and sulfadimethoxine. Most sulfa drugs are readily absorbed from the gastrointestinal tract and rapidly accumulate in the blood and organs in bacteriostatic concentrations. Sulfa drugs are used to treat erysipelas, cystitis, and various forms of angina. Phthalylsulfathiazole and sulfaguanidine are poorly absorbed from the gastrointestinal tract and remain in high concentrations in the intestine for a relatively long time. They are used to treat dysentery and other intestinal infections.

Sulfa drugs act on the human body as well as on microorganisms. Some sulfa drugs, for example, tolbutamide, can lower blood sugar levels and are therefore used to treat diabetes mellitus. Sulfa drugs can cause such side effects as allergic reactions, nausea, vomiting, a decrease in white blood cells, neuritis, and renal dysfunction. They are used only when prescribed by a physician.