The FDA, which regulates the world's biggest drug market, said it would give more definitive advice in three months. An agency spokeswoman declined to comment.

The "early communication" is part of a new FDA effort to notify the public and doctors sooner about potential drug risks. It comes after criticism of a slow response in dealing with Merck & Co's now-recalled Vioxx painkiller and safety fears tied to GlaxoSmithKline's Avandia treatment for diabetes.

Nexium is AstraZeneca's top-selling product and was the world's No. 2 best-selling drug in the world in 2006, according to pharmaceutical information firm IMS Health. AstraZeneca reported $5.2 billion in Nexium sales for last year.

The company and Procter & Gamble Co, which sells over-the-counter Prilosec, have said they agreed with the FDA's early assessment of no risk.

"These patients have two common problems: a need for (a Prilosec-type drug), and heart disease," said Dr. Michael Weinstein, who is on the board of the American Gastroenterology Association. "Both are very common, and the fact that they occasionally occur together is not surprising."

Weinstein, a gastroenterologist in private practice in Washington, is waiting for peer-reviewed proof of a link before he changes his prescription habits.

"The fact that there may be some association does not mean cause," he added. "I think that is the way most of us look at these kinds of reports."

The FDA has said both studies in question may have been skewed, in part because surgical patients tended to be younger and less likely to have risk factors for heart problems.

Fourteen other studies of patients treated for up to two years did not suggest a higher risk of heart problems with Prilosec, the FDA said.

Nexium and Prilosec are proton pump inhibitors that reduce the amount of acid produced by the stomach. Both are sold by prescription to treat acid reflux and ulcers. Prilosec also is sold over the counter for frequent heartburn.

Merck shares profits from the drugs.

More than 1 billion patients have taken the drugs, AstraZeneca said. Prilosec is known generically as omeprazole, and Nexium is known as esomeprazole.

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