A new study released Sunday has again raised questions about the effectiveness of Merk and Co’s cholesterol drug Zetia and its sister drug Vytorin. In this most recent study, Zetia failed to shrink buildups in artery walls. It was compared to a newer drug, Niaspan, which did so significantly. Zetia users also suffered more heart attacks and other problems although the numbers of these events were too small to draw any conclusions.

Zetia “has been on the market for about seven years and we still haven’t proven that it improves clinical outcomes,” said Roger Blumenthal, preventive cardiology chief at Johns Hopkins University. The results will be “very influential” in getting more doctors to turn to Niaspan, he said.

Doctors say that the study results are not conclusive and that users should not stop taking there prescribed medication. In a report from Reuters, Dr. Paul Armstrong of the University of Alberta in Edmonton said he received an email from a patient early Monday morning asking if she should stop taking Zetia even though it was having the intended effect on her LDL levels.

Doctors say not to worry if the medicine is having its intended affects, but to be cautious if any side affects are experienced and don’t hesitate to contact your doctor for consultation.