In FASS, it is stated that there is an interaction between erythromycin and terfenadine and therefo

Fråga: In FASS, it is stated that there is an interaction between erythromycin and terfenadine and therefore these drugs may not be combined. Is there a risk of interaction between erythromycin and Clarityn (loratadine)? A male patient is being treated with erythromycin 500 mg, two tablets twice daily. He has also been prescribed loratadine 1x10 mg daily.

Sammanfattning: No reports could be found concerning an interaction between loratadine and erythromycin. Erythromycin is known to interact with terfenadine, by inhibiting its P450 IIIA4 metabolic pathway. Since it is not known whether loratadine is metabolised by the same cytochrome isoenzyme, the possibility of an interaction between erythromycin and loratadine cannot be excluded.

Svar: Both terfenadine and loratadine belong to the so-called low-sedating H1 antagonists (1). Terfenadine has been associated with adverse cardiovascular reactions such as prolongation of the QT interval and ventricular arrhythmias including Torsades de Pointes (polymorphic ventricular tachycardia). Patients with increased terfenadine concentrations, including drug interaction-induced elevations, are particularly at risk (1,2). Terfenadine is metabolised by the cytochrome P450 IIIA4, the metabolic activity of which is known to be inhibited by erythromycin. The reduction in terfenadine metabolism and accumulation of the parent drug is supposedly responsible for the cardiac toxicity. Also, terfenadine appears to interact with a number of other drugs that inhibit cytochrome P450 IIIA4, for example ketoconazole (3).

After an extensive literature search, no reports could be found concerning an interaction between loratadine and erythromycin. According to the manufacturer (4), no interactions of loratadine with erythromycin have been reported so far. It is not known whether loratadine is metabolised by the same cytochrome isoenzyme as terfenadine. 1 Hansten, Horn, Drug interactions and Updates. 1971-; page 586-588 2 Drugline nr 09756 (year 1993) 3 Zimmermann M, Duruz H, Guinand O, Broccard O, Levy P, Lacatis D, Bloch A: Torsades de Pointes after treatment with terfenadine and ketoconazole. Eur Heart J 1992; 13: 1002-1003 4 Personal communication Anders Lundman, Schering-Plough, Stockholm