[Incidence and severity of drug interactions in the elderly: a prospective study of 639 patients].

The medicinal treatments of 639 patients aged over 65 were recorded on admission to hospital. The mean number of drugs consumed was 4.4 +/- 2.8 per patient. Drug interaction was found in 37 p. 100 of the patients on the basis of data published in the Vidal dictionary. The prevalence of interactions increased with the number of drugs prescribed. The medicinal families most frequently involved were digitalis derivatives, antiarrythmic agents, diuretics, anticoagulants and psychotropic drugs. Thirty patients (4.7 p. 100) presented with a side-effect that was directly ascribable to an interaction. Among the iatrogenic adverse reactions 11 were life-threatening, including 8 cases of severe dysrhythmia and 3 cases of gastrointestinal haemorrhages. Altogether, one-third of all iatrogenic disorders were consecutive to a drug interaction. Simple precautions would have considerably reduced the frequency of such side-effects.