Popular asthma drug Singulair has come under scrutiny recently after reports of suicidal behavior, suicide, and mood changes among patients who are using the drug. Merck says that none of the patients in clinical trials reported these types of problems, but because it's such a serious issue, the FDA is investigating. Singulair's labels will be updated to inform patients of new side effects, including tremors, anxiousness, depression, and suicide.

Singulair contains the active ingredient montelukast. This drug blocks the action of leukotrienes, which are released as part of an allergic or inflammatory reaction and create swelling of the respiratory tract. It's prescribed to people whose asthma does not stay under good control with other medications, and sometimes to people who suffer nasal allergies.