It doesn't come as much of a surprise, but it hurts nonetheless.

According to Merck (NYSE: MRK), clinical trials testing vorapaxar in stroke patients were stopped last week because the drug seems to be increasing bleeding in the brain.

Bleeding is the No. 1 issue with anticlotting drugs. After all, the ones that are the best at keeping the blood from clotting to avoid heart attacks are likely to overshoot and cause excessive bleeding. With anticlotting drugs, there tends to be a small therapeutic window, and vorapaxar seems to have hit the wall instead of going through the window -- at least in stroke patients. Eli Lilly's (NYSE: LLY) Effient ran into the same issue, and it hasn't been able to compete well against Plavix from Bristol-Myers Squibb (NYSE: BMY) and sanofi-aventis (NYSE: SNY).

One trial is still continuing without patients that have previously had a stroke. It's possible that whatever increased bleeding seen in patients who haven't had a stroke is overwhelmed by the positive effects of vorapaxar. For instance, a drug that has a 1% increase in deaths from bleeding would theoretically be justified by a 20% decrease in deaths from heart attacks. And vorapaxar is used in combination with Plavix, so it wouldn't have to compete with the entrenched leader.

The reality is that doctors might not be quite so excited to prescribe vorapaxar even if the combined rate of death works out in favor of the drug. Telling a patient's family that the drug a doctor prescribed killed him because of bleeding is a lot harder than not prescribing vorpaxar and telling the family that the patient died of a heart attack because Plavix wasn't good enough.

Some doctors won't worry about the issue if the math works out in the average patient's favor, but severe side effects like bleeding in the brain won't be summarily dismissed. Biogen Idec (Nasdaq: BIIB) and Elan's (NYSE: ELN) Tysabri is clearly the best multiple sclerosis drug out there, but growth has stagnated because of the fear of deadly brain infection called progressive multifocal leukoencephalopathy (PML).

Merck already bled out last week when its shares took a major hit. The confirmation this week should help investors realize they're unlikely to be able to pencil back in meaningful sales from vorpaxar.

Jim Gillies points out how to benefit from Merck's downturn with call options.

Elan is a Motley Fool Rule Breakers pick. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.