U.S. drug regulators are reviewing data from a halted clinical trial of Sanofi's key heart drug Multaq to determine how it may affect patients taking it for approved indications, according to Reuters.

The FDA's probe joins that of the European Medicines Agency that last week widened its review of Multaq's safety after the trial for people with permanent atrial fibrillation revealed severe cardiovascular events in some patients.

Multaq is currently approved, based on another trial, for treating people with non-permanent atrial fibrillation, an irregular heartbeat condition. It has been sold in Europe since July 2009 and in the U.S. since early last year.

The drug is seen as an important driver for Sanofi to see it through patent expiries on multibillion-dollar drugs such as cancer treatment Taxotere and blood thinner Plavix.

Europe's drugs watchdog first started reviewing the risks associated with the drug in January following two cases of liver failure in patients on the drug. The FDA at the time issued a warning about the liver failures.

Concerns about the medicine increased last week, however, when the French drug maker reported a higher rate of adverse events and hospitalizations with Multaq when compared with placebo in patients with permanent atrial fibrillation. The drug maker stopped the trial.

The EMA said its review of Multaq had been extended to assess the new clinical study findings and determine need for any further action this week. It has delayed that deadline until September.

The FDA recommended patients taking Multaq consult their doctors whether they should continue doing so and reminded doctors to not prescribe Multaq for permanent atrial fibrillation.