A multiple sclerosis drug sold by Swiss drug giant Novartis AG came under safety scanner, after an European agency said it would investigate the drug following reports of heart problems and at least one death related to it.

Last month, a person reportedly died in the U.S. within 24 hours of taking the drug while reports of 10 other deaths have only deepened concerns related to the drug's side effects.

Currently, it is not clear if the deaths were caused by the drug or not, the European Medicines Agency has said.

The drug, Gilenya, was approved as the first-ever oral treatment for multiple sclerosis in the U.S. and it came with a known side effect of bradycardia or slow heart rate associated with the first dose.

It got EU marketing approval last year and so far more than 30,000 people have used the drug worldwide.

While the review is going on, the European Medicines Agency advised doctors to increase their level of monitoring of patients after the first dose of the medicine is administered by the use of electrocardiograms and measuring blood pressure and heart rate every hour. The agency expects to complete the review by March.

The drug is touted to have a billion dollar market worldwide, but reports of serious heart problems associated with the drug have become a major concern for the drug maker, which is set to lose a large chunk of its revenue as some of its popular drugs face patent expiry.

According to Reuters data, Gilenya has the potential to earn $1.8 billion in annual sales by 2016.

Multiple sclerosis is a poorly understood nervous disorder that affects 2.5 million people worldwide, mostly women, which can cause numbness, paralysis and loss of vision.