POSSIBLE INTERACTIONS WITH TERFENADINE OR ASTEMIZOLE

Concurrent use of terfenadine or astemizole with erythromycin or ketoc onazole can prolong the QT interval and produce potentially fatal vent ricular arrhythmias. We examine the frequency and patterns of concurre nt prescribing and suggest methods to reduce the incidence of serious drug interactions. By retrospectively reviewing Oregon Medicaid prescr iption claims data over 22 months, we determined the frequency of conc urrent prescribing of terfenadine or astemizole with macrolide antibio tics 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 t erfenadine or astemizole with macrolide antibiotics or ketoconazole. M ost of these episodes (94%) involved terfenadine. The frequency of con current use increased more than threefold from 1991 to 1992. Although patients received prescriptions from different physicians in 48% of th ese episodes, they used different pharmacies only 3% of the time. We d emonstrate that terfenadine use is extensive and increasing, thus incr easing the possibility of serious interactions, and many physicians ma y remain unaware of this potential. Effective prospective screening by pharmacists could dramatically reduce the incidence of concurrent pre scribing. Physicians must be aware of the potential for these drug int eractions, avoid prescribing these medications concurrently, and consi der these interactions in the evaluation of syncope and cardiac arrhyt hmias.