Which antibiotic has been found to be toxic and is primarily used for the treatment of tuberculosis?

• A. Penicillin
• B. Cycloserine
• C. Vancomycin
• D. Bacitracin

Answer: (B)

Cycloserine is a toxic, second-line antitubercular drug used mainly for drug-resistant tuberculosis. It acts as a D-alanine analog that inhibits two enzymes involved in bacterial cell-wall synthesis—alanine racemase and D-alanine:D-alanine ligase—disrupting peptidoglycan formation. This mechanism helps target Mycobacterium tuberculosis when first-line drugs aren’t effective, but the compound’s neurotoxicity is a major red flag; patients can experience headaches, confusion, seizures, and other CNS effects, which is why its use is reserved for difficult TB cases. The other antibiotics listed don’t share this profile: penicillin, vancomycin, and bacitracin do not form the standard backbone of TB therapy and are used for other types of bacterial infections, with varying safety and efficacy in TB.