The serum levels and therefore the therapeutic effectiveness of the tetracyclines can be markedly reduced or even abolished by antacids containing aluminium, bismuth, calcium or magnesium. Other antacids, such as sodium bicarbonate, may also reduce the bioavailability of some tetracyclines. Even intravenous doxycycline levels can be reduced by antacids.
As a general rule none of the aluminium, bismuth, calcium or magnesium- containing antacids should be given at the same time as the tetracycline antibacterials. If they must be used, separate the dosages by 2 to 3 hours or more, to prevent their admixture in the gut. This also applies to quinapril formulations containing substantial quantities of magnesium (such as Accupro), although the interaction is less pronounced, and didanosine tablets formulated with antacids. H2-receptor antagonists do not interact, and they may therefore be a suitable alternative.
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