DRUG interactions with quinidine are likely to be clinically important, since quinidine is used in the treatment and prophylaxis of potentially serious arrhythmias, and since there is a relatively narrow range between the therapeutic and toxic concentrations.1 An interaction between quinidine and the anticonvulsant drugs primidone and phenobarbital has led to a reduction in the half-life of quinidine of approximately 50 per cent.1 Enhanced quinidine elimination after hepatic-enzyme induction by the anticonvulsants was suggested as the likely mechanism. A cautious approach in determining the dosage of quinidine was urged for patients who were receiving this drug and had any change . . .