Which drug is primarily used as monotherapy for latent TB infection but is not recommended as monotherapy for active TB?

• A. Rifampin
• B. Isoniazid
• C. Ethambutol
• D. Pyrazinamide

Answer: (B)

Isoniazid is used as monotherapy for latent TB infection because it effectively prevents reactivation by targeting dormant bacilli, which is why a long, single-drug course (typically 6–9 months) is the standard approach for LTBI. In latent infection, the bacteria aren’t actively multiplying, so a single agent like isoniazid can reduce the risk of progressing to active TB.

For active TB, though, monotherapy is not recommended because the bacilli are actively dividing and can quickly develop resistance if only one drug is used. Active TB requires a multi-drug regimen to effectively kill the bacteria and prevent resistance. In practice, isoniazid is included as part of the initial multi-drug backbone (along with rifampin, pyrazinamide, and ethambutol) during the intensive phase, followed by continuation therapy with multiple drugs. Auxiliary notes: pyridoxine (vitamin B6) is often given with isoniazid to prevent neuropathy, and liver function should be monitored due to hepatotoxicity risk.