What are the effects of tetracyclines when taken with antacids or iron salts?

Tetracyclines should not be taken with antacids or iron salts because they can form complexes that render the tetracyclines unabsorbable. This interaction decreases the bioavailability of tetracyclines, making them less effective.

When tetracyclines are consumed with antacids (except sodium bicarbonate) or iron salts, they can react with divalent ions present in these substances, leading to the formation of tetracycline–calcium complexes. This reaction significantly reduces the absorption of tetracyclines in the gastrointestinal tract, which in turn lowers their therapeutic effectiveness. To avoid this issue, it is recommended that tetracyclines be taken at least one hour before or two hours after meals or the intake of these substances.