Reports of cardiac arrhythmias caused by unusually high blood levels of 2 antihistamines (terfenadine and astemizole) are best explained by

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Terfenadine and astemizole can cause cardiac arrhythmias when their metabolism is inhibited, leading to high blood levels. Ketoconazole, a potent inhibitor of the cytochrome P450 enzyme system, can significantly increase the levels of drugs metabolized by this pathway, including terfenadine and astemizole, thus explaining the cardiac arrhythmias.

Reports of Cardiac Arrhythmias Caused by Unusually High Blood Levels