European researchers found that the anti-retroviral Ziagen (also known as Abacavir), included as part of many anti-AIDS regimens worldwide, almost doubled patients’ risks of heart problems. Also, didanosine (Videx) increased the heart attack risk by about 50 percent.

GlaxoSmithKline PLC makes Ziagen, while Bristol-Meyers Squibb makes Videx.

Jens D. Lundgren of the University of Copenhagen and colleagues analyzed data from more than 33,000 people infected with the AIDS virus in Europe, the U.S. and Australia to study the long-term effects of five AIDS drugs. The patients were followed for up to five years.

The researchers found that in the 517 patients, who had heart attacks, 192 had already taken Ziagen and 124 had recently taken Videx. Those given Ziagen had twice the chances of a heart attack compared to patients on other AIDS drugs, while those on Videx had a 50 percent higher chance. However, the risk disappeared six months after patients stopped taking the drugs.

The study found no increased heart attack risk in patients on the other drugs in the study, zidovudine (AZT), stavudine (Zerit) or lamivudine (Epivir). These drugs block an enzyme that the AIDS virus needs to multiply.

The findings of the study were denied by Dr. Didier Lapierre of GlaxoSmithKline who said the company’s own analysis of 54 studies did not suggest an increased risk of heart attack from Ziagen, but added: “GSK takes the finding seriously and is committed to understanding these data more fully and to communicating openly with treating physicians and regulatory agencies globally.”

The researchers suggested the increased risk of heart attack from Ziagen and Videx could be attributed to a cardiovascular inflammatory response to the drugs, but pointed out more research is needed in this area. They added that patients already susceptible to heart problems, such as smokers and the obese were most at risk.

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

“These anti-retrovirals are wonderful and lifesaving, but they do have toxicity problems. It may be that we can continue to see them, but we need to be aware of their long-term problems,” Dr. Charlie Gilks, director of AIDS treatment and prevention at the World Health Organization, who was not involved in the study, said, quoted by the USA Today.

The FDA already put a notice on their Web site stating until evaluation on the two drugs is complete, health-care providers should evaluate the potential risks and benefits for each drug. It is likely that once the evaluation is complete, the labels on Ziagen and Videx will change to show these new risks.

HIV infects about 33.2 million people worldwide, according to the World Health Organization, including about 1 million in the U.S. There is no vaccine against HIV, which mutates constantly to evade drugs, keeping researchers on a constant quest for new treatments.

The research, funded by the European Medicines Agency, a regulatory group, which solicited contributions from makers of AIDS drugs for studies on their long-term effects, was published Tuesday in the journal The Lancet.