Which drug is commonly used for TB prophylaxis after exposure?

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

Answer: (A)

Post-exposure TB prophylaxis aims to prevent a latent TB infection from progressing to active disease. The drug most commonly used for this purpose is isoniazid, given as preventive therapy after exposure to reduce the risk of developing active TB. Isoniazid works by inhibiting synthesis of mycolic acids in the mycobacterial cell wall once it is activated by the bacterial enzyme KatG, effectively suppressing latent bacteria that may have been acquired during exposure. The usual course is several months (commonly around 6 to 9 months), which has been shown to significantly cut the chance of later active TB. Because of potential liver toxicity, liver function monitoring is important, and vitamin B6 is often advised to prevent neuropathy in at-risk individuals.

In some situations, rifampin can be used as an alternative for post-exposure prophylaxis (for example if isoniazid cannot be used due to intolerance or resistance issues), but isoniazid remains the standard, most widely used option in routine prophylaxis. Pyrazinamide or ethambutol are not standard choices for simple post-exposure prophylaxis.