WASHINGTON - The Food and Drug Administration said Thursday it is investigating a possible link between Merck’s best-selling asthma drug, Singulair, and suicide. FDA said it is reviewing reports of mood changes, suicidal behavior and suicide in patients who have taken the popular drug also used for allergies. Merck has updated the drug’s labeling four times in the past year to include information on a range of side effects: tremors, anxiousness, depression and suicidal behavior.