Clinicians should consider prescribing azithromycin to older patients taking calcium-channel blockers for hypertension in favor of other macrolide antibiotics, study results suggest.

David Juurlink, MD and colleagues from the Sunnybrook Research Institute in Toronto, analyzed hospital admission rates due to hypotension or shock among 999,234 Ontario adults aged 66 years and older that had been prescribed one calcium channel blocker (verapamil, diltiazem, nifedipine, amlodipine or felodipine) between Apr. 1, 1994 and Mar. 31, 2009.

Data indicated that 7,100 individuals met these criteria, 176 of which had been prescribed a macrolide antibiotic during study risk and control intervals.

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The researchers determined that among the macrolide antibiotics patients had been prescribed, erythromycin increased risk for hospitalization the most (OR=5.8, 95% CI: 2.3-15.0), followed by clarithromycin (OR=3.7, 95% CI: 2.3-6.1). Azithromycin, however, was not associated with an increased risk (OR=1.5, 95% CI: 0.3-2.8).

“Our findings highlight the consequences of an underappreciated yet avoidable drug interaction involving medications used by millions of patients each year,” the researchers wrote in the Canadian Medical Association Journal.

They believe inhibition of cytochrome P450 isoenzyme 34A is the mechanism of action, as erythromycin and clarithromycin are known to inhibit these molecules, whereas azithromycin does not.

“Clinicians should be aware of the potential interactions between these drugs,” the researchers wrote. “When a macrolide is required, preferential use of azithromycin should be considered in patients already receiving a calcium-channel blocker.”