If you're going to invest in the high-risk, high-reward stakes of heart drugs, you've got to be ready for the crashes. Merck's (NYSE: MRK) investors found that out the hard way yesterday, when its shares fell nearly 7% after the company announced that one of its most promising pipeline drugs, vorapaxar, might be a dud.

On the recommendation of the Data and Safety Monitoring Board overseeing the two clinical studies that Merck is conducting with vorapaxar, one of them -- codenamed Tracer -- is being stopped, and another trial called TRA-2P will be scaled back to exclude patients who have previously had a stroke.

Merck didn't say exactly why the trials were being stopped, but many people -- this Fool included -- think excessive bleeding is the likely culprit. Bleeding is often a problem with these anticlotting agents because it's hard to hit the small window where you keep a clot from forming but don't go too far and cause bleeding.

Even if a reduction in heart attacks by vorapaxar makes up for the increased bleeding, doctors might not prescribe it. Eli Lilly's (NYSE: LLY) Effient has had a horrible time competing with Bristol-Myers Squibb (NYSE: BMY) and sanofi-aventis' (NYSE: SNY) Plavix, likely because of increased bleeding seen in clinical trials.

The knockdown yesterday was well deserved. Merck -- and Schering-Plough, before Merck bought it -- spent a lot of money on vorapaxar in hopes of it becoming a multibillion-dollar drug. That seems unlikely at this point.

But the loss isn't a complete knockout for Merck, either. Investing in heart drugs, which require large clinical trials, may be risky, but Merck has a decent pipeline of drugs. If it can get a hit, especially for a drug such as odanacatib in the large osteoporosis market, Merck's shares could be headed in the other direction.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.