On Saturday, drug-maker Merck & Co provided details about the last month-announced results of the tests of its experimental blood thinner `vorapaxar', revealing that despite the impressive effectiveness of the blood clot drug in bringing about a significant reduction in cardio vascular incidents in heart patients, worrisome bleeding was also witnessed in people who had suffered a stroke.

According to Merck, while the experimental drug, when combined with standard care, proved effective in bringing down the instances of cardio vascular events in heart patients, it also led to increased bleeding in some patients.

When Merck presented limited data from the study in February this year, the company had said that vorapaxar was proving to be an advantageous drug for heart patients. However, the drug-maker has now revealed that patients who had been administered vorapaxar were also experiencing excessive bleeding.

The Merck revelation, based on the results of the biggest heart-drug studies conducted thus far, implies that the vorapaxar blood clot preventer has shown mixed results; and has thus led to concerns whether the drug-maker will go ahead and seek regulatory approval for the drug.

Noting that the vorapaxar trial shows that there is "clearly a tradeoff here between the protective benefit and the risk of bleeding," Dr. David Morrow - who is a cardiologist from Brigham & Women's Hospital in Boston, and the leader of the trial - said: "Should the drug be approved, he said the challenge for doctors would be to withhold it from patients at likely higher bleeding risk."