OBJECTIVE: To evaluate protein-bound cyanocobalamin (vitamin B12) absorption before and after omeprazole (Prilosec) therapy in healthy male volunteers. DESIGN: Clinical trial in which each volunteer served as his own control. SETTING: Outpatient department of a university medical center. PARTICIPANTS: Ten healthy, male volunteers 22 to 50 years old. INTERVENTION: Each participant had a modified Schilling test (protein-bound cyanocobalamin) and a gastric analysis, as well as measurements of serum vitamin B12, gastrin, and folate levels. Five patients were then randomly assigned to take either 20 mg or 40 mg of omeprazole daily. After 2 weeks of omeprazole therapy, these tests were repeated. MEASUREMENTS: The modified Schilling test, gastric analysis, serum gastrin level, folate level, and cyanocobalamin level. RESULTS: At the end of the 2-week treatment period, cyanocobalamin absorption decreased from 3.2% to 0.9% (P = 0.031) in participants receiving 20 mg of omeprazole daily. In patients taking 40 mg of omeprazole daily, cyanocobalamin absorption decreased from 3.4% to 0.4% (P < 0.05). CONCLUSIONS: Omeprazole therapy acutely decreased cyanocobalamin absorption in a dose-dependent manner.