Antacids are commonly used drugs which are considered inert and free of pharmacological effect by many patients and physicians. When administered with other drugs, antacids can alter absorption or excretion of these agents, at times reducing their plasma level and effect and in other cases causing toxicity.Although many antacid-drug interaction studies have been performed in animals orin vitroand others show only minor effects in man, several clinically important interactions and their mechanisms have been verified in man. Antacids which reduce drug dissolution or bind drugs in the gastrointestinal tract have been shown to inhibit absorption and reduce efficacy of tetracyclines, digoxin, phenytoin and certain psychotherapeutic agents. Aluminium hydroxide gel delays gastric emptying, delaying isoniazid absorption and lowering blood levels. By elevating urine pH, antacids cause accelerated excretion of acidic drugs like salicylate and per contra may cause retention of basic drugs like quinidine or amphetamine.While the efficacy of many drugs may not be affected by antacids, the absorption or excretion of other drugs which have not yet been studied are undoubtedly altered by antacid administration. It is therefore advisable, where possible, to administer other drugs at least a half to I hour before antacid ingestion to ensure consistent absorption and effect. Long-term antacid ingestion should be avoided when other drugs are taken, unless antacid use is clinically indicated and is of proven benefit.