The powerful drugs needed to treat Aids patients may make them more vulnerable to a heart attack, research suggests.
HIV drugs are very effective
A study of 23,000 patients by researchers in Copenhagen found the heart attack risk rose by 26% per year for people taking the drugs.
However, it is unclear whether HIV itself, or the drugs heighten risk.
The finding, published in the New England Journal of Medicine, contradicts previous research, and the researchers say more work is needed.
They stress the overall rate of heart disease among the patients in the study was low.
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Only 126 patients had a heart attack during the 18-month study period.
And they accept that their research by no means provides conclusive proof.
"The absolute risk of myocardial infarction was low and must be
balanced against the marked benefits from antiretroviral treatment," they said.
A combination of different types of anti-HIV drugs taken at the same time has proved to be a very effective way to stall progression of the disease - effectively turning it from a certain killer into a chronic condition with which people can live for many years.
Writing in the same journal, HIV experts Dr Peter Sklar and Dr Henry Masur, said: "Antiretroviral therapies have been among the miracles of recent decades.
"Yet we must work toward mitigating the toxic effects that have the potential to diminish the quality and duration of patients' survival over the long term."
However, they also stressed that HIV therapy is complex, and that strong evidence is needed before a change in treatment programmes is considered.
Nearly 45 million people around the world live with HIV - most of them in developing countries where they have no medical treatment.