Which is an interaction/complication of H2 blockers?

• A. Increased sleepiness
• B. Increased warfarin and phenytoin levels
• C. No interactions
• D. Increases vitamin B12 absorption

Answer: (B)

H2 blockers can affect how other drugs are processed in the body, especially when a blocker like cimetidine is used. Cimetidine inhibits several liver enzymes (CYP450), which slows the breakdown of drugs such as warfarin and phenytoin. When their metabolism is reduced, their levels in the blood rise, increasing the risk of warfarin-related bleeding and phenytoin-related toxicity. That’s why increased levels of warfarin and phenytoin is the best described interaction.

In contrast, these drugs don’t typically increase vitamin B12 absorption; long-term acid suppression can actually reduce B12 absorption. And while some sedating effects or interactions with other medicines can occur, the classic well-documented interaction among H2 blockers is the elevation of warfarin and phenytoin levels due to enzyme inhibition.