Breakfast was observed to reduce doxycycline absorption significantly. Absorption of tetracycline occurs in the stomach and the upper small intestine. Absorption of tetracyclines has been reported to be impaired by milk products, aluminum hydroxide gels, sodium bicarbonate, calcium and magnesium salts, laxatives containing magnesium and iron preparations. The mechanisms responsible for decreased absorption appear to be chelation and an increase in gastric pH. ... In view of these results, it is advisable to instruct the patients to take doxycycline on an empty stomach.