ENHANCEMENT OF DRUG ABSORPTION BY ANTACIDS - AN UNRECOGNIZED DRUG-INTERACTION

Antacids are widely used for many disorders. The potential of antacids to interact with other concomitantly ingested drugs is well recognise d. These interactions usually result in reduced or delayed absorption of the affected drug. However, this is not always the case. In contras t to aluminium hydroxide, magnesium hydroxide and sodium bicarbonate c an enhance the absorption of some drugs. For example, magnesium hydrox ide can increase the rate and sometimes even the extent of absorption of certain nonsteroidal anti-inflammatory drugs (e.g. tolfenamic acid, mefenamic acid and ibuprofen), sulphonylurea antidiabetic agents [e.g . glipizide, glibenclamide (glyburide) and tolbutamide] and the oral a nticoagulant dicoumarol (bishydroxycoumarin). These weakly acidic drug s art nonionised at gastric pH, but are sparingly water soluble. Eleva tion of the gastric pH by administration of magnesium hydroxide or sod ium bicarbonate increases the solubility and absorption of such sparin gly water soluble agents. Chelate formation may be involved in the inc reased absorption of dicoumarol by magnesium hydroxide. In combination antacids containing both aluminium hydroxide and magnesium hydroxide, the absorption enhancing effect of magnesium hydroxide seems to be co unterbalanced by the opposing effects of aluminium hydroxide. The clin ical significance of increased drug absorption is not clear. However, accelerated and enhanced absorption of analgesic drugs may be benefici al when rapid pain relief is desired. In contrast, an unexpectedly inc reased hypoglycaemic or anticoagulant effect may be potentially danger ous. Therefore, a knowledge of the potential effect of antacids on the absorption of other drugs is clinically important.