The Food and Drug Administration has said that Merck's Singulair, which is the brand name for the substance montelukast, may raise the risk of suicide. The drug regulator is currently looking at data involving mood changes linked with the popular allergy and asthma drug.

The agency said it received around three to four suicide reports since last October but a causal relationship between montelukast and suicide has not been established yet. As a result, the FDA asked Merck to investigate the matter and see through further studies if such a link does exist.

"We have no indication that anything about the mechanism of Singulair is consistent with these events," said George Philip, director of research and product development, according to AP. "But because suicide is a life-threatening event we thought it was important to provide this information in the product label."

Risks of tremors, anxiousness, depression and suicidal behavior are already in the drug's label as a result of several updates last year. The company said that none of the 11,000 patients in their 40 studies commited suicide, and the FDA investigation was triggered by reports only, not studies.

The issue may also affect similar drugs such as AstraZeneca's Accolate and Critical Therapeutics's Zyflo, which work similarly to Singulair. Their labels do not warn about the risk of suicide.

Montelukast, used by millions of Americans, reduces the bronchoconstriction and inflammation. Its specificity enables it to be combined with other asthma drugs. It is available as oral tablets, chewable tablets, and oral granules. The drug, marketed in the US as Singulair, is one of Merck's top-selling products, with 2007 sales of $4.3 billion.