Health implications of drug interactions

The awareness of the digoxin-quinidine interaction in the mid-70s led to increased interest in drug interactions. Much has been published on the subject in the form of interaction reports, handbooks, lists, cards, discs etc. However, only a few interactions in the literature have clinical significance and can be remembered by thinking in terms of groups, pharmacokinetics and probabilities. It is important to realise that drugs in a given class will have similar properties. Thus, phenylbutazone, like any other nonsteroidal anti-inflammatory drug (NSAID), is likely to potentiate the effects of warfarin. A knowledge of drug pharmacokinetics is also essential; a highly protein-bound drug may displace another when they are administered together. Remembering those drugs which induce liver enzyme formation will prevent their co-administration with drugs which have a critical dose-range. If a general practitioner can remember the few drugs in clinical practice with a narrow therapeutic index, he can consult a handbook before anything else is prescribed. Some interactions will rarely be encountered in daily general practice, i.e. those associated with tuberculosis treatment or anesthetic agents. Other interactions, e.g. the interaction between antihypertensive treatment and over-the-counter medication containing phenylpropanolamine, are more common. While most drug interactions can be avoided by thinking in terms of groups, pharmacokinetics and probabilities, some learning by rote is required, e.g. the potential for heart failure with concomitant beta-blocker and nifedipine therapy. In general, a schematic approach using thinking in terms of groups, pharmacokinetics and probabilities will prevent most clinically significant drug interactions; the rest can be avoided by consulting appropriate handbooks and specialists.