Popular asthma and allergy drug Singulair is being investigated by Canadian and U.S. regulators for a possible link to mood changes and suicidal behaviour in patients.

The U.S. Food and Drug Administration on Thursday ordered Merck & Co. -- Singulair's manufacturer -- to review all patient data after receiving several reports of suicide.

Meanwhile, the FDA says patients should not stop taking the prescription drug before talking to their doctor.

Merck officials point out the FDA investigation is based on three to four suicide reports the agency has received since last October -- and not on clinical studies, through which drug safety is routinely evaluated.

The company also said of the 11,000 patients enrolled in 40 Singulair trials, none have committed suicide.

The FDA has not established a "causal relationship" between Singulair and suicidal behaviour, an agency spokesperson said.

Health Canada is also investigating the possible link between the drug and suicidal behaviour, spokesperson Paul Duchesne told The Canadian Press.

Duchesne said he wasn't aware of suicide reports linked to the drug in Canada, and said patients should see their doctors if they have concerns about taking Singulair.

According to a posting on the FDA's website, it could take up to nine months before the agency's scientists complete their investigation and draw any conclusions.

Merck has updated the labelling for Singulair four times in the past year to include a number of reported side effects, including tremors, anxiousness, depression and suicidal behavior.

However, the company's director of research and product development said there is no indication that the drug is linked to these events.

"But because suicide is a life-threatening event we thought it was important to provide this information in the product label," said George Philip.

Results from clinical trials of asthma patients showed the most common side effects were headache, flu, abdominal pain and cough.

With a report from The Associated Press