Shares of Merck & Co. are falling after the drugmaker says it has halted one late-stage trial of a potential blood thinner and will immediately stop giving the drug in another study to patients who have had a stroke.

The Whitehouse Station, N.J., company is studying vorapaxar for the prevention of cardiac events. Patients with acute coronary syndrome will stop taking it in one study because it reached a pre-determined number of medical issues like cardiovascular deaths, heart attacks or strokes.

A second study will continue with about 75 percent of its patients, minus those who had a stroke either before or during the trial.

The company's shares are down $2.03, or 5.5 percent, to $35.12 in morning trading.