Calcium channel blockers and antibiotics might be a bad combination in the elderly, study says

Calcium channel blockers prescribed to lower blood pressure and macrolide antibiotics to treat infections can combine to sharply reduce blood pressure in the elderly, leading to an increased risk of hospitalization and other problems, Canadian researchers reported Monday. The combination of drugs probably reduces blood pressure in younger patients as well, but represents a bigger risk in the elderly, who are already at increased risk of falls, said Dr. David Juurlink of the Sunnybrook Research Institute in Toronto, who led the study reported in the Canadian Medical Assn. Journal. Physicians already knew about the risk of combining the two families of drugs, he said, but the new study is the first to quantify the risks.

The macrolide antibiotics, which include erythromycin, clarithromycin and azithromycin, are among the most widely prescribed antibiotics, with an estimated 66 million prescriptions written in the United States in 2008. The calcium channel blockers are a popular family of drugs for reducing blood pressure that includes such well-known products as Norvasc, Procardia, Plendil, Adalat, Cardizem, Dilacor and Tiazac.

Humans have an enzyme called cytochrome P450 3A4 that helps to metabolize calcium channel blockers and clear them from the system. Erythromycin and clarithromycin -- but not azithromycin -- block the activity of this enzyme, allowing the drugs to build up to high levels in the body, reducing blood pressure to undesirably low levels.

Juurlink and his colleagues used publicly available health records of elderly patients in Ontario who had been prescribed a calcium channel blocker between April 1, 1994 and March 31, 2009 and who had been hospitalized for hypotension. Among the 7,100 hospitalized patients, they found that those who had received erythromycin were six times as likely to be hospitalized, while those who had received clarithromycin were four times as likely. Those who received azithromycin were at no increased risk, however.

One shortcoming of the study, experts noted, was that they were not able to determine whether the patients drank grapefruit juice, which can also interfere with the activity of the enzyme in question.

Juurlink’s advice to physicians treating a patient with an infection: If they are receiving blood pressure medication, be safe and start with azithromycin.