Interaction Between a Macrolide Antibiotic and a Second-Generation Antihistamine: A Case Report

Authors: Valliammai Valliyappan, Ashok Veerappan, Nishkkriyaa Gopal

ABSTRACT

Drug–drug interactions are a common yet often overlooked cause of adverse drug reactions. We report a case of a 30-year-old man who developed QT interval prolongation after receiving erythromycin and loratadine concurrently for an upper respiratory tract infection. The event was attributed to the additive QT-prolonging effect of erythromycin at therapeutic dose and reduced hepatic metabolism of loratadine due to CYP3A4 inhibition by erythromycin, further increasing QT prolongation risk. Discontinuation of erythromycin and loratadine led to rapid clinical improvement and normalisation of electrocardiographic findings. This case underscores the importance of careful prescribing, medication review, and awareness of pharmacokinetic interactions.

Keywords: CYP3A4, Drug interaction, Erythromycin, Loratadine, QT prolongation

Citations: IeJSME 2026 Vol 20 (1): 57-59