The U.S. Food and Drug Administration (FDA) is working with the drug company Merck to investigate a possible link between Merck's asthma and allergy drug Singulair and suicide risk associated with it. Meanwhile, "patients should not stop taking Singulair before talking to their doctor," the FDA said in a statement, adding that doctors should monitor patients for suicidal behavior and mood changes.

The federal agency said that the investigation may take nine months. Currently there is no proof that Singulair can lead to mood changes, suicidal behavior and suicide in patients.

Singulair is used to treat asthma and the symptoms of allergic rhinitis (sneezing, stuffy nose, runny nose, itching of the nose) and to prevent exercise-induced asthma. It's in a class of drugs called leukotriene receptor antagonists.

The review was prompted by three to four suicide reports FDA has received since last October. However Merck officials stressed none of the 11,000 patients enrolled in 40 Singulair trials has committed suicide.

The Whitehouse, N.J.-based company has recently added reports of suicide to Singulair's label, which already listed suicidal thinking and behavior as reported side-effects. Singulair's labeling has changed four times in the past year to include information on a range of reported side-effects, which also include tremors, anxiousness and depression.