The U.S. Food and Drug Administration said Friday it was updating the label of Merck & Co.'s (MRK) Singulair and similar asthma medications to discuss reports of neuropsychiatric events such as agitation, depression, insomnia and suicidal thinking.

The new labeling will apply to Accolate, by AstraZeneca PLC (AZN), and Zyflo, by Cornerstone Therapeutics Inc. (CRTX). All three products fall into a drug class known as leukotriene modifiers. Leukotrienes are chemicals the body releases in response to an inflammatory stimulus such as breathing in an allergen.

The FDA said in a posting on its Web site that "patients and healthcare professionals should be aware of the potential for neuropsychiatric events with these medications."

Singulair, first approved in the U.S. in 1998, is Merck's top-selling product and had $4.3 billion in sales in 2008 and is the dominant drug in its class. The product is approved for use in children and adults as an asthma and allergy treatment. Accolate and Zyflo are approved as asthma treatments.

In a statement, Merck said it was "confident in the efficacy and safety of Singulair" and that it would work with the FDA to revise the label.

Merck noted it previously updated the post-marketing section of Singulair's label to discuss post-marketing reports of neuropsychiatric events.

The FDA said it requested manufacturers to include a precaution in the drug-prescribing information discussing such events, which is considered a strengthening of Singulair's current label and new information on the other two drug labels.

The FDA announced a safety review of Singulair last year after receiving reports of mood and behavior changes in patients who used Singulair. The agency asked Merck and the other two drug makers to submit all available clinical trial information, which the agency reviewed along with post-marketing reports.

The agency said some of the post-marketing reports involving neuropsychiatric events "included clinical details consistent with a drug-induced effect."

The FDA said reported neuropsychiatric events include agitation, aggression, anxiousness, dream abnormalities and hallucinations, depression, insomnia, irritability, restlessness, suicidal thinking and behavior, suicides and tremor. The agency said neuropsychiatric events "were not commonly observed" in the clinical trial data submitted by the manufacturers, but said the studies weren't designed to look for such events. The agency said sleep disorders such as insomnia were reported more frequently in clinical studies of all three drugs compared to patients taking placebos.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com