Which combination may reduce vitamin B12 absorption?

• A. Cimetidine and neomycin
• B. Warfarin and calcium
• C. Grapefruit juice and statin
• D. Vitamin C and iron

Answer: (A)

Vitamin B12 absorption requires several coordinated steps: stomach acid frees B12 from dietary protein, B12 then binds intrinsic factor produced by stomach cells, and the complex is absorbed in the terminal ileum. Medications that lower stomach acidity reduce the release of B12 from protein, making it harder to form the B12–intrinsic factor complex and be absorbed. Cimetidine is an H2 blocker that decreases gastric acidity, which can blunt this initial release and transfer process. Neomycin is an antibiotic that can disrupt gut bacteria and mucosal health, potentially altering the intestinal environment where B12 absorption occurs. Together, these two drugs can impair B12 absorption at different points in the pathway, increasing the risk of deficiency.

Other combinations listed don’t typically influence B12 absorption in the same way: calcium can interfere with iron absorption, not B12, and the grapefruit-statin and vitamin C–iron pairs involve different nutrient or metabolic interactions rather than the B12 absorption steps.