Several commonly coadministered drugs interfere significantly with the pharmacokinetics or pharmacodynamics of cardiac glycosides. Only a few of these interactions (e.g. amiodarone, propafenone, quinidine) take place consistently, and although their extent may vary in individual patients, digitalis dosage adjustments should be made to avoid underdigitalization or toxicity. In other instances the appearance of clinically significant interactions depends on individual pharmacokinetic/metabolic characteristics (e.g. erythromycin, tetracycline), and the result cannot be anticipated on clinical grounds. Some interactions are controversial, having not been confirmed by all studies; others have been shown only in healthy volunteers but lack the definition of their relevance in the context of disease states. In view of the possible impact on the individual patient, close clinical monitoring (which may be supplemented with evaluation of digitalis plasma concentration) is recommended when prescribing cardiac glycosides with other therapeutic agents for which the possibility of an interaction has been reported.