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.

Welcome to pharma investing. It isn't just about the dividend.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.