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Of the many varieties of drug interactions, which occur when the disposition or actions of one drug are changed by another, only a few are serious or potentially fatal. A representative outline of some of these illustrates the problem. Precipitant drugs are those which produce the interaction, and object drugs are those whose effects are changed. The interactions which are usually significant are those which alter the metabolism, involve renal excretion, or change the effects of the object drug, especially when the object drug has a low therapeutic index (cardiovascular drugs, anticoagulants, drugs acting on the brain, hypoglycemic drugs, hormones, and cytotoxic drugs). Warfarin toxicity, for example, is produced by aspirin, phenylbutazone, and azapropazone. The dosage requirements of warfarin are reduced by chloramphenicol, ciprofloxacin and other quinolones, erythromycin and some of the other macrolides, metronidazole and other imidazoles, tetracyclines, amiodarone, cimetidine (but not ranitidine), and fibrates. Potassium-depleting drugs can potentiate the action of digoxin, and the elimination of digoxin can be reduced by amiodarone, propafenone, quinidine, and verapamil. Combined oral contraceptives can lose effectiveness through the interaction of carbamazepine, griseofulvin, phenytoin, or rifampicin, which increase estrogen metabolism. In addition, broad-spectrum antibiotics such as ampicillin or tetracyclines also reduce contraceptive effectiveness by altering gut absorption. Even a single drink of an alcoholic beverage may be dangerous to people taking antidepressants, antihistamines, antipsychotic drugs, benzodiazepines, or lithium. Antihistamines suffer inhibited metabolism in the liver if taken in conjunction with the antifungal imidazoles and some of the macrolide antibiotics. Cardiotoxicity of antihistamines is also enhanced by drugs with similar cardiotoxic effects. Lithium potentiation is enhanced by the new serotonin-reuptake inhibitors, and lithium excretion can be reduced by diuretics or fluoxetine. When drugs such as antifungal imidazoles, azapropazone, or phenylbutazone are permitted to inhibit the metabolism of sulphonylureas, hypoglycemic effects are enhanced and, if unnoticed, may cause brain damage. Fibrates should not be combined with HMG-CoA reductase inhibitors because of the increased risk of myopathy. Patients taking non-selective monoamine oxidase inhibitors should avoid amine-containing foods and drugs such as matured cheeses, meat, yeast extracts, some wines, unfresh protein, and cold-curing medications. The metabolism of azathioprine is inhibited by allopurinol, and this combination requires a reduced dosage of azathioprine. Mercaptopurine, used in the treatment of leukemia, is also a metabolite of azathioprine. Sources of comprehensive information on drug interactions are 1) the "British National Formulary," appendix 1; 2) Chapter 10 of "The Oxford Textbook of Clinical Pharmacology and Drug Therapy"; and 3) a monograph by Stockley entitled "Drug Interactions."


Journal article


The Practitioner

Publication Date





789 - 791