The Food and Drug Administration has approved a highly anticipated blood thinner from Eli Lilly after more than 18 months of review.

The approval makes Lilly's Effient the first real competition to the blood thinner Plavix, the world's second-best selling medication made by Sanofi-Aventis and Bristol-Myers Squibb.

The FDA delayed its decision on Effient multiple times as reviewers weighed the drug's benefits versus its risks

A study of over 13,000 patients conducted by Lilly found that Effient prevents more heart attacks than Plavix, but also causes more internal bleeding.

The FDA says Effient will carry a boxed warning to alert physicians to the risks of potentially fatal bleeding.