TRENTON, N.J. (AP) — Sanofi-Aventis SA said Thursday its drug Multaq, which treats irregular heart rhythms, received marketing approval from the Food and Drug Administration, which turned it down three years ago.

Multaq is intended to treat atrial flutter and atrial fibrillation, related heart disorders in which the organ's upper chambers occasionally beat rapidly and ineffectively, sometimes reducing blood flow through the body. The disorder often is associated with high blood pressure and heart disease, and can cause strokes and death.

"It's the first new anti-arrhythmic therapy that's been approved in over 10 years," Dr. Paul Chew, Sanofi's chief U.S. medical and scientific officer, said in an interview.

Multaq also is the first drug for atrial fibrillation that has been shown to reduce hospitalizations due to cardiovascular problems.

"Multaq represents a therapeutic innovation for treatment of ... atrial fibrillation," Dr. Norman Stockbridge, head of heart and kidney drugs at the FDA's Center for Drug Evaluation and Research, said in a statement.

Multaq, or dronedarone, will be sold in 400-milligram tablets intended to be taken twice per day. Sanofi-Aventis plans to launch the drug this summer. Officials said the price has not yet been determined.

Atrial fibrillation is the most common heart rhythm disorder. The company estimates there are 2.5 million atrial fibrillation patients in the U.S., and another 4.5 million in the European Union, where the company expects a decision on whether the drug will be approved by the end of this year.

In August 2006, the Food and Drug Administration rejected the French drugmaker's request for approval. That was after a widely publicized study called ANDROMEDA found patients in the group taking Multaq were twice as likely to die as patients in the comparison group.

"Most, maybe all drug companies, might have thrown in the towel," Chew said.

But that study involved patients with severe heart failure, a condition where the heart no longer pumps enough blood that also puts patients at risk of atrial fibrillation. Many patients in the study did not even have the irregular rhythm.

So Sanofi did another study, called ATHENA, that included 4,628 patients with atrial fibrillation but not severe heart failure.

This time, results were positive: Multaq helped reduce hospitalization and death from heart-related problems by 24 percent, and it lowered the risk of both ischemic and hemorrhagic stroke by 34 percent.

An FDA cardiology panel in March recommended approval.

But on Thursday, the FDA said the drug will come with a black box warning, its most severe, stating Multaq can cause severe complications, including death, in people with recent severe heart failure and so should not be used in those patients. The most common side effects were fatigue, loss of strength, diarrhea, nausea and vomiting.

U.S. shares of Sanofi-Aventis fell 64 cents, or 2.1 percent, to $29.70 Thursday, when virtually all major drugmakers and the broader markets fell sharply. The day before, the FDA said it was reviewing the safety of Sanofi's synthetic insulin, Lantus, due to an inconclusive new study indicating a possibility it slightly raised risk of cancer.

The stock has traded between $23.95 and $37.11 over the past 52 weeks.