Federal health officials are requiring new labels on popular asthma drugs like Merck's Singulair that highlight language about suicidal behavior, depression and anxiousness seen in a small number of patients.

After 15 months of investigation, the FDA is asking Merck, AstraZeneca and Critical Therapeutics to raise warnings about psychiatric problems reported by a handful of patients taking their drugs.

A spokeswoman for Merck says the language already appears in Singulair's label, but will be raised to the "precautions" section. It's now listed in a section about various side effects reported by patients.

Singulair was Whitehouse Station, N.J.-based Merck's best-selling product last year, with U.S. sales of $3.5 billion.