Ziagen, one of the most widely-used AIDS drugs, may boost the risk of a heart attack, a long-term study published online on Wednesday by The Lancet suggests.
The authors say the implications of this are troubling, although they caution further research is needed.
Ziagen -- lab name abacavir -- is a common component in the "cocktail" of anti-retroviral treatments that keep the AIDS virus at bay. It is included among generic drugs to help HIV-infected people, especially in Africa.
Researchers led by Jens Lundgren of the Centre for Viral Diseases and in Copenhagen looked at data from an ongoing probe into the health of 33,300 people with HIV being followed at 212 clinics in Europe, the United States and Australia.
This study, which goes by the initials of D:A:D, is considered to be of an exceptionally high quality, given the frequent checkups given to the patients and the depth and scope of data about their health.
The Lundgren team were looking for any evidence of heart attacks among patients who used Ziagen, a less-used drug called Videx (lab name didanosine) as well as AZT (zidovudine), d4T (stavudine) and 3TC (lamivudine).
The drugs fall into a broad pharmaceutical category called nucleoside reverse transcriptase inhibitors (NRTIs), which block the activity of an enzyme that the human immunodeficiency virus (HIV) needs in order to reproduce.
The investigators say they found no evidence of any risk with AZT, d4T and 3TC.
But the risk of a heart attack among people taking Videx was 49 percent higher than among individuals who did not use the drug, and the risk among those using Ziagen was 89 percent higher.
These risks were high only while people were taking Ziagen or Videx. After individuals stopped using the drugs for six months, they no longer ran a higher risk.
In a commentary, also carried by The Lancet, US experts James Stein and Judith Carrier noted that, in absolute terms, the risk of a heart attack was not high.
A total of 517 heart attacks were recorded during the period that was studied, which translates into only 3.3 per 1,000 "patient years," as the benchmark for duration in medical studies is called.
Even so, the higher risk for people with a known hazard of a heart attack -- smokers, or people with a family history of cardiovascular disease, for instance -- "cannot be ignored," they said.
Among these patients, one additional heart attack could be expected for every 11 treated with Ziagen or every 20 treated with Videx for five years.
The British firm GlaxoSmithKline (GSK), which makes Ziagen, said Lundgren's findings were "unexpected" but would be taken seriously. It said, though, that its own analysis of 54 studies into the drug did not suggest any increased risk.
Previous research has found a link between heart attacks and a different class of antiretrovirals, called protease inhibitors. This is the first to dwell specifically on the same question among drugs in the NRTI class.
The British AIDS campaign group, the Terrence Higgins Trust, said the study "highlights a risk that hasn't been identified before.
"When deciding the best treatment for each individual this research should be taken into account along with other risk factors like family history, smoking and lack of exercise," Roger Pebody, the trust's treatment advisor, told AFP.
"Some people taking abacavir [Ziagen] may choose to change their treatment, but others may decide with their doctor that it's still the best option for them."