dropped nearly 4% Thursday after the Food and Drug Administration said it was reviewing clinical trials that indicate the ulcer drugs Nexium and Prilosec might raise the risk of heart attack or heart failure.
However, the FDA emphasized that its review of Nexium -- AstraZeneca's biggest-selling drug -- and its older cousin, Prilosec, doesn't mean the agency has determined that these drugs cause problems.
The FDA's preliminary conclusion is that collectively, these data do not suggest an increased risk of heart problems for patients" treated with either drug, the agency said. The "FDA does not believe that healthcare providers or patients should change either their prescribing practices or their use of these products at this time."
In addition to ulcers, Nexium and Prilosec also treat gastroesophageal reflux disease, or GERD, which causes food and stomach acid to back up into the esophagus. This disease is the subject of the clinical trials being reviewed by the FDA.
The FDA said it will complete its analysis in three months. Canada's Health Department said it is examining the clinical trial results as well.
Prilosec and Nexium, both manufactured by AstraZeneca, are sold via prescription.
Procter & Gamble
sells an over-the-counter version of Prilosec for frequent heartburn, but the FDA is focusing on the prescription-strength drug.
AstraZeneca's American Depositary shares recently were slipping $1.91, or 3.7%, to $49.13 on more than four times their average daily volume.
The FDA said AstraZeneca alerted the agency and foreign regulators in late May about the two clinical studies on GERD patients. Some received Nexium or Prilosec, while others underwent surgery to control GERD.
The results "raised concerns that long-term use of Prilosec or Nexium may have increased the risk of heart attacks, heart failure, and heart-related sudden death" in patients receiving drugs compared with those undergoing surgery, the FDA said. The agency said AstraZeneca has provided more information since May 29, including results from other clinical studies.
The Prilosec study ran 14 years. The difference between the two groups of patients was seen within the first year, and continued over time, the FDA said.
The Nexium study, which is still in progress, has tracked patients for five years. The initial data "suggested a difference between treatments in the rate of cardiovascular events," the FDA said. The agency said AstraZeneca recently submitted an updated report showing cardiovascular risks were similar between the drug group and the surgery group.
The FDA added that weaknesses in both studies' design -- including the defining and documenting of heart problems -- makes a safety evaluation for both drugs "difficult."
Nexium is AstraZeneca's No. 1 drug, accounting for about 18% of its revenue in the first half of the year. For the six months ended June 30, Nexium had $2.6 billion in worldwide sales, including $1.72 billion in the U.S. Prilosec had $577 million in worldwide sales.