Pharmaceuticals designed to reduce stomach acid, such as the proton-pump inhibitors (PPIs), are among the most widely sold drugs in the world. One PPI, omeprazole (Prilosec), has been found to lower the bioavailability of vitamin C by 12% after 28 days of treatment, independent of dietary intake. The probable mechanism of vitamin C reduction, intragastric pH elevated into alkalinity, would apply to all other PPI drugs, though not necessarily to doses of PPIs low enough to keep the stomach slightly acidic. In another study, 40 mg/day of omeprazole lowered the fasting gastric vitamin C levels from 3.8 to 0.7 µg/mL. Aspirin may also inhibit the absorption of vitamin C.
