LONDON -- A drug commonly used to fight AIDS appears to nearly double the risk of a heart attack, researchers said.

In a study published online in The Lancet medical journal, European researchers said that the antiretroviral abacavir, included as part of many anti-AIDS regimens worldwide, almost doubled patients' chances of heart problems. The lesser-used drug didanosine also increased the heart attack risk by about 50 percent.

Abacavir, also known as Ziagen, is made by GlaxoSmithKline PLC. Didanosine, or Videx, is made by Bristol-Meyers Squibb.

While experts said that doctors should be aware of the increased heart attack risk, they did not recommend that patients abandon the drugs.

"These antiretrovirals are wonderful and lifesaving, but they do have toxicity problems," said Dr. Charlie Gilks, director of AIDS treatment and prevention at the World Health Organization. "It may be that we can continue to use them, but we need to be aware of their long-term problems," Gilks said. He was not connected to the study.

Jens D. Lundgren of the University of Copenhagen and colleagues analyzed data from more than 33,000 people with HIV in Europe, the United States and Australia. Patients were enrolled in the study from 1999 to 2005. Researchers looked for any heart problems that occurred until February 2007.

In the 517 patients who had heart attacks, 124 had recently taken didanosine and 192 had recently taken abacavir.

Patients on Ziagen had twice the chances of a heart attack compared to patients on other antiretrovirals. Those on Videx had a 50 percent higher chance. But the risk disappeared six months after patients stopped taking the drugs.

The study was funded by the European Medicines Evaluation Agency, which received money to study the long-term effects of anti-AIDS drugs made by various pharmaceutical companies, including Bristol-Meyers, GlaxoSmithKline, F. Hoffman-La Roche, Gilead Sciences, Merck & Co. and Pfizer Inc.

The findings could influence how AIDS patients are treated globally, as health authorities like WHO reconsider their treatment guidelines. Ziagen and Videx are currently recommended by WHO for people with HIV worldwide.

"In developed countries, doctors have 24 different antiretrovirals to choose from if one isn't appropriate. But if that happens in resource-poor countries, it is not so simple," Gilks said. "WHO will have to review what the likely implications of using these drugs is on a large scale and if we should consider alternatives."

GlaxoSmithKline said their own analysis of their database of about 14,600 HIV patients, did not support the Lancet study's conclusions. "We were unable to show any increased risk in heart attacks," said Gwenan White, a company spokeswoman. "Abacavir does not increase lipids or glucose levels," she said. She called the Lancet study results "inconclusive."

Lundgren said patients already susceptible to heart problems, such as smokers and the obese, were most at risk. In people with HIV, heart attacks do not appear to be more deadly than in the normal population, though some doctors suspect that once people with HIV have a heart attack, they are more likely than others to have another.

Gilks said more information was needed about how the drugs worked in the body, particularly in children. "If we don't understand the mechanism of this cardiac risk, are we putting kids at risk for the next 30 or 40 years?"

As HIV patients on antiretroviral therapy continue to live longer, experts said they would probably see more of the rarer side effects emerge. "No drug is risk-free," Lundgren said. "For all patients, it's a matter of finding the right balance."