A federal advisory panel meets Friday to review an experimental pill that supporters say could dramatically affect stroke prevention and provide a much needed boost for the drug's developer, U.K.-based AstraZeneca ( AZN). But the drug, Exanta, has an Achilles heel that gives experts pause and could delay FDA approval or result in restricted use. Clinical studies show that patients taking the blood thinner have a significantly higher rate of an elevated liver enzyme vs. the market-dominating pill, something that could signal liver damage. "We believe that investor expectations for Exanta may be too high given the high incidence of liver side effects seen in the clinical development program," said Tim Anderson, a Prudential Equity Group analyst, in a research report in late July when he maintained a neutral rating on the stock. He believes the FDA may take a "cautious stance" on the drug. (He doesn't own shares of AstraZeneca; his firm doesn't have an investment banking relationship with the company.) The FDA says that drug-induced liver injury is the top cause of acute liver failure, a rare disease that can produce critical illness in a few days, cause death or necessitate a liver transplant. It's the leading reason drugs are removed from the market and why the FDA restricts use of some drugs and requires special monitoring of patients. And it's a major reason why experimental drugs don't reach the marketplace. FDA approval would herald the first new anticoagulant pill in more than 60 years to challenge the prevailing pill, sold by its generic name warfarin and best known by the brand name Coumadin. Warfarin is a crucial medication for preventing strokes because it prevents blood clots in the legs and lungs; clots in patients who have had a heart valve replaced with a mechanical valve; and clots associated with an irregular rapid heartbeat called atrial fibrillation. The drug is also given to heart attack patients to lower the risks of death, another heart attack and stroke.