U.S., Canada reviewing Nexium, see no heart risk

But the U.S. Food and Drug Administration and Health Canada said their preliminary analyses did not show a higher heart risk and doctors should not change prescribing practices at the moment.

AstraZeneca shares closed 4 percent lower on the New York Stock Exchange. Nexium is the company's top-selling drug and was the second best selling drug in the world in 2006, according to drug information firm IMS Health. The company reported $5.2 billion in Nexium sales last year.

One Prilosec study and analyses from an ongoing Nexium study had raised concerns that long-term use might increase the risk of heart attacks, heart failure and heart-related sudden death when compared with surgery, the FDA said.

The FDA said it was still evaluating those studies plus other data and planned to finish the review in three months.

"Based on everything we know now, FDA's preliminary conclusion is that the observed difference in risk of heart attacks and other heart-related problems seen in early analyses of the two small long-term studies is not a true effect," an FDA statement said.

Doctors should not change prescribing habits and patients should not alter use of the drugs, the FDA said.

Regulatory agency Health Canada said its preliminary review also had found no evidence to confirm a cardiovascular risk.

Even so, analysts said the issue could hurt sales.

"This will create a shadow of a doubt about the safety of these drugs," Raymond James analyst Mike Krensavage said.