Shares of Merck & Co,. (NYSE: MRK) this morning after the pharma giant said that clinical studies for vorapaxar, an experimental anti-clotting medication, were halted for patients who have had a previous stroke, thereby possibly reducing its future use.

Merck shares are down 5.53 percent as of 10:28 a.m.

Researchers at Brigham and Women’s Hospital and the Duke Clinical Research Institute said use of the drug was inappropriate for previous strike patients.

The drug was once expected to generate at least $5-billion in annual sales.