Possible interactions with terfenadine or astemizole.

Concurrent use of terfenadine or astemizole with erythromycin or ketoconazole can prolong the QT interval and produce potentially fatal ventricular arrhythmias. We examine the frequency and patterns of concurrent prescribing and suggest methods to reduce the incidence of serious drug interactions. By retrospectively reviewing Oregon Medicaid prescription claims data over 22 months, we determined the frequency of concurrent prescribing of terfenadine or astemizole with macrolide antibiotics or ketoconazole. From 1991 to 1992, terfenadine use increased by 29%, with a seasonal peak in June of each year. Terfenadine was one of the most prescribed medications from March through July 1992. During the 22 months reviewed, there were 122 episodes of concurrent use of terfenadine or astemizole with macrolide antibiotics or ketoconazole. Most of these episodes (94%) involved terfenadine. The frequency of concurrent use increased more than threefold from 1991 to 1992. Although patients received prescriptions from different physicians in 48% of these episodes, they used different pharmacies only 3% of the time. We demonstrate that terfenadine use is extensive and increasing, thus increasing the possibility of serious interactions, and many physicians may remain unaware of this potential. Effective prospective screening by pharmacists could dramatically reduce the incidence of concurrent prescribing. Physicians must be aware of the potential for these drug interactions, avoid prescribing these medications concurrently, and consider these interactions in the evaluation of syncope and cardiac arrhythmias.