1. one of a group of antibiotic compounds derived from cultures of Streptomyces bacteria. These drugs, which include chlortetracycline, doxycycline, oxytetracycline, and tetracycline, are effective against a wide range of bacterial infections. They are usually given by mouth to treat various conditions, including respiratory-tract infections, syphilis, and acne. Side-effects such as nausea, vomiting, and diarrhoea are fairly common. In addition, suppression of normal intestinal bacteria may make the patient susceptible to infection with tetracycline-resistant organisms. Tetracyclines should not be administered after the fourth month of pregnancy and their use should be avoided in young children to prevent unsightly staining of the permanent teeth.
2. a particular antibiotic of the tetracycline group. Trade names: Achromycin, Tetrabid.