The FDA issued a warning yesterday that patients taking Merck's (NYSE:MRK) Singulair have reported depression and even suicidal thoughts. There's no evidence that the drug itself is causing depression or suicide, but because suicide is so serious, the FDA felt compelled to tell the public.

I don't believe there's any reason for investors to become despondent over the news, though. It's worth noting that the warning wasn't based in a placebo-controlled trial showing that there was a higher incidence of depression among patients taking Singulair. Instead, the notice was issued because a handful of suicides among patients taking Singulair have been reported to the FDA.

Singulair's label already indicates that there have been postmarketing reports of suicidal thinking, but Merck has agreed to be more vigilant about pointing out the findings to doctors as well as evaluate safety data from clinical trials for evidence of suicidal thoughts.

The FDA is also reviewing postmarketing reports from AstraZeneca's (NYSE:AZN) Accolate, as well as from Zyflo, which is marketed by Critical Therapeutics (NASDAQ:CRTX) and Dey, a new subsidiary of Mylan Labs (NYSE:MYL). Those two drugs are in the same class as Singulair, but it's not clear whether there have been suicides reported with those drugs.

These types of reports have become more common. The FDA is increasingly vigilant about safety these days, after postmarketing side effects surfaced for Merck's Vioxx and GlaxoSmithKline's (NYSE:GSK) Avandia. That means investors in drug companies will need to sift through the warnings like this one -- which should have minimal effect on sales -- to find ones that may turn out to be more serious.

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