UPDATE 3-US FDA asks for caution on Merck asthma drug

* FDA requests precaution on Singulair, Accolate, Zyflo

* Some patients reported psychiatric problems

* Merck says remains confident in Singulair's safety (Adds analyst comment, sales figures)

By Lisa Richwine

WASHINGTON, June 12 (Reuters) - Merck & Co Inc's (MRK.N) top-selling drug Singulair and similar medicines should come with a caution about reports of psychiatric problems in some patients, U.S. regulators said on Friday.

The Food and Drug Administration said it had requested a precaution in the prescribing instructions for Singulair, AstraZeneca Plc's (AZN.L) Accolate and Cornerstone Therapeutics Inc's (CRTX.O) Zyflo and Zyflo CR. A precaution is a less serious alert than a warning.

Problems that were reported include agitation, aggression, suicidal thinking, suicide, depression, insomnia and irritability, an FDA notice said.

Psychiatric problems were uncommon in clinical trials of the drugs but those data were limited because the studies were not designed evaluate those issues, the FDA said.

Some post-approval reports of psychiatric events "included clinical details consistent with a drug-induced effect," the FDA added.