Another day, another head-to-head trial to evaluate. And this one is a biggie. AstraZeneca (NYSE:AZN) is trying to grab some of the $8.6 billion market currently held by Bristol-Myers Squibb (NYSE:BMY) and sanofi-aventis' (NYSE:SNY) anticlotting drug, Plavix.

Head-to-head trials can be risky for drug companies because they allow doctors to clearly distinguish which drug is the winner, but it looks like AstraZeneca came out on top. The company said that its anticlotting drug, Brilinta, was able to prolong the time to the first cardiovascular event -- a heart attack or stroke, for instance -- when compared to Plavix. AstraZeneca is holding the full data set -- the press release didn't include any data -- for a presentation at a cardiology conference in August.

The devil is sometimes in the details, and for Brilinta the details could be its safety profile rather than by how much it beat Plavix. Eli Lilly's (NYSE:LLY) prasugrel also looked better than Plavix, but the Food and Drug Administration has delayed approving the drug, possibly because the agency isn't sure what to do about the excessive bleeding seen in some patients.

Entrenched leaders can be overthrown -- Johnson & Johnson's (NYSE:JNJ) Tylenol and Wyeth's (NYSE:WYE) Advil took plenty of market share from Bayer's Aspirin. However, the side effect profile needs to justify any increase in efficacy that a new player brings.

AstraZeneca's investors today should be cautiously optimistic about the top-line results. Brilinta could be a blockbuster for the company, but it may need some brilliant marketing to compete with prasugrel, which may have a year's head start, and generic versions of Plavix set to hit the U.S. market in 2011.

Our counting abilities are brilliant:

Johnson & Johnson is an Income Investor recommendation. To see how dividend-paying stocks can offer both secure income and the opportunity for growth, take a free look at this newsletter with a 30-day trial.

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