Researchers say ongoing use of clarithromycin - a commonly prescribed antibiotic - is linked to an increased risk of cardiac death.
Clarithromycin belongs to a class of drugs known as macrolide antibiotics. It is used to treat an array of bacterial infections, including pneumonia, bronchitis and ear, sinus, skin and throat infections.
According to the research team - including senior investigator Anders Hviid of the Statens Serum Institut in Copenhagen, Denmark - macrolide antibiotics increase the duration of the heart muscle's electrical activity - known as the QT interval - which can lead to abnormal heart rhythm (arrhythmia), a known risk factor for stroke and sudden cardiac arrest.
But the team notes that it is unclear how individual macrolide antibiotics affect the heart.
With this in mind, they set out to assess how clarithromycin and roxithromycin - another macrolide antibiotic - influence the risk of cardiac death.
Clarithromycin use linked to 37 additional cardiac deaths per 1 million courses
Hviid and colleagues analyzed more than 5 million Danish adults ages 40-74 between 1997 and 2011. All adults were receiving 7-day treatment courses with one of three antibiotics: clarithromycin, roxithromycin and penicillin V - an antibiotic with no link to heart problems.
The researchers excluded individuals with serious illness who may have had high risk of death at study baseline.
During the study period, there were 285 cardiac deaths. Of these, 32 occurred with continued use of roxithromycin, and 18 occurred with continued use of clarithromycin.
However, when the researchers accounted for factors such as patients' age, sex, risk of death at study baseline and use of other medication, they found that continued use of clarithromycin increased the risk of cardiac death by 76%, compared with ongoing penicillin V use.
This represents an absolute risk of an additional 37 cardiac deaths for every 1 million courses of clarithromycin, compared with every 1 million courses of penicillin V. Furthermore, the increased risk of cardiac death with continued clarithromycin use appeared to be more prominent among women.
After adjusting for the same factors, the team found that past or continued use of roxithromycin did not increase the risk of cardiac death.
Commenting on their findings, the researchers say:
"Our study expands on the available knowledge of the cardiac safety of macrolides, being the first large-scale population-based observational study to show significantly increased cardiac risk with clarithromycin and the relative cardiac safety of roxithromycin."
They stress, however, that the absolute risk of cardiac death with ongoing clarithromycin use is small, therefore doctors should not yet change how they prescribe this antibiotic to patients.
But they say that given the widespread use of clarithromycin, their findings require "urgent confirmation."
Hviid and colleagues note that their study is subject to some limitations. For example, they were unable to obtain patient information about certain factors that may influence the risk of cardiac death, such as smoking and body mass index (BMI).
The researchers were also unaware of what type of infections the antibiotics were prescribed for. "However," they say, "the fact that clarithromycin and roxithromycin have essentially identical indications should reduce concerns about the results being influenced through an effect of infection rather than the prescribed treatment."
Furthermore, the team points out that the overall rate of cardiac death in this study was low, therefore their findings were based on few events. "The power to detect differences in subgroup analysis may have been limited," they say.
Medical News Today recently reported on a study led by researchers from the NYU Langone Medical Center in New York, NY, which claims antibiotic exposure early in life may increase the risk of later-life obesity and metabolic abnormalities.