Adding Zetia (ezetimibe) to another anti-cholesterol drug, the statin Zocor (simvastatin), did not provide additional protection from harmful artery-blocking fatty plaques, according to a study reported by Forbes.com and other media outlets. Though people living with HIV were not included in the study, those who are currently taking either Zetia, or Vytorin (a combination pill that contains both ezetimibe and simvastatin), to treat high cholesterol may want to discuss the results of this trial with their health care provider.

Previous studies found that Zetia modestly lowers LDL—the bad cholesterol—in the general population and in people living with HIV. In the ENHANCE study, reported by Forbes, HIV-negative people with high cholesterol saw their LDL levels drop 40 percent more than those who took just Zocor. But the ENHANCE study was designed to determine whether Vytorin could go a step further and reduce the buildup of fatty plaques (arteriosclerosis) that can lead to stroke, heart attacks and other cardiovascular problems.

Contrary to expectations, researchers found that people taking Vytorin did not produce fewer fatty plaques than those taking Zocor alone. In fact, those taking Vytorin actually had more arteriosclerosis than those taking just Zocor.

Unlike Zocor and other statins, which block the production of cholesterol in the liver, Zetia blocks the intestine from absorbing cholesterol in food.

Though the drugs’ makers, Merck and Schering-Plough, point out that numerous studies have found that lowering LDL does reduce the risk of heart attack and death, Allen J. Taylor, MD, head of cardiology at Walter Reed Army Medical Center, says the results “do not bode well” for larger studies that aim to prove Zetia and Vytorin prevent heart attacks, strokes and deaths better than other medicines.