The Food and Drug Administration said Thursday it is investigating a possible link between Merck’s allergy drug Singulair and suicide.

FDA said it is reviewing reports of mood changes, suicidal behavior and suicide in patients who have taken the drug, which was Merck’s best-selling product last year.

In the past year Merck has updated the drug’s labeling four times to include information on tremors, anxiousness, depression and suicidal behavior reported in some patients.

Singulair had sales of $4.3 billion last year.

Shares of Merck & Co. Inc. fell 10 cents to $44.60 in late morning trading.