January 18, 2011

Antibiotics, Blood Pressure Med Mix Can Be Harmful

According to a study published on Monday, older adults on blood pressure drugs known as calcium channel blockers could suffer dangerous drops in blood pressure if they are given certain antibiotics.

Researchers found that patients on calcium channel blockers who were given the antibiotics erythromycin or clarithromycin were at an increased risk of being hospitalized for dangerously low blood pressure.

A third antibiotic in the same class was not linked to the risk.  The researchers say that it should be chosen over the other two antibiotics for people on calcium channel blockers.

The three bacteria-killing drugs are known as macrolide antibiotics.  Over 66 million prescriptions of these drugs were filled in the U.S. in 2008.

Calcium channel blockers are among the most commonly used long-term medications.  They include drugs like amlodipine, felodipine, nifedipine and diltiazem.

"It's not far-fetched that a person would be on a calcium channel blocker and macrolide at the same time," Dr. David M. Juurlink, the senior researcher on the new study, told Reuters.

He said the problem with erythromycin and clarithromycin is that they inhibit an enzyme vital to metabolizing calcium channel blockers.

Blood levels in the calcium channel blocker spike as a result, which then sometimes leads to severe drops in blood pressure.

The researchers looked at medical records for over 999,000 Ontario residents age 66 and up who were on a calcium channel blocker at some point between 1994 and 2009.

During that time, 7,100 were hospitalized for severe drops in blood pressure.  Of those patients, 131 were on a macrolide antibiotic in the week before their hospitalization.

Juurlink's team found that erythromycin treatment was linked to a six-fold increase in the risk of hospitalization for low blood pressure. 

The team said he could not give a precise figure for the absolute risk of severe hypotension from the drug combination, due to the study design.  However, he said that it was "probably very low" for any particular person.

Juurlink said that it appears to be an avoidable risk.

"Doctors should be aware of the risk, and know that there is an alternative out there," he said. "If you have to prescribe a macrolide antibiotic to someone on a calcium channel blocker, it makes sense to give azithromycin."

According to Juurlink, other classes of blood pressure drugs do not seem to be susceptible to the effects of the two antibiotics.

The current study focused on older adults because they are most vulnerable to the consequences of drops in blood pressure. 

Low blood pressure can deprive the organs of blood and oxygen, leading to shock and potentially fatal organ damage.

"Younger patients are better able to tolerate a drop in blood pressure," Juurlink said.

However, he also said that azithromycin would still be the wise choice for younger people on calcium channel blockers as well.

He also pointed out that erythromycin and clarithromycin can boost blood levels of cholesterol-lowering statins.  He said that the concern would be increasing the risk of serious muscle damage, a known side effect of statins.

Juurlink said it would be wise for people on statins to take azithromycin over the other two macrolides.

Another of the researchers on the study has served as a consultant to Zithromax maker Pfizer Inc.  All three macrolide antibiotics are also available as generics.


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