When the FDA announced in 2009 that Pfizer’s smoking-cessation drug Chantix would need to carry a restrictive "black box" warning label, the move didn't really surprise the market, according to a Reuters report.

"When this drug launched, a lot of people expected a blockbuster drug, but then the reports of the side effects started coming in," Damien Conover, an analyst with Morningstar, told Reuters Health.  The FDA's response came after hundreds of reports of erratic behavior and several suicides.

Experts say all of this could have been avoided had the company's clinical trials included people with depression.

More than 40% of smokers are depressed, compared to just 7% of the general population, according to statistics from the Centers for Disease Control and Prevention and the National Institute of Mental Health.

Doctors know that depression often coincides with other disorders, such as addiction and heart disease. Yet clinical trials for these conditions routinely exclude patients with depression.  This strategy may benefit drug companies during the early testing stages, but it can backfire once the treatment is released into the real world.

Of 38 actively recruiting large-scale phase III clinical trials for smoking cessation, 21 excluded people with mental illness, and 10 did so for depression specifically.  Late-stage studies of heart disease were the same: about 17% excluded patients with mental illness.

Clinical trials are supposed to mirror the potential patient population as much as possible, but drug companies frequently screen out participants with a history of psychiatric illness, citing safety concerns and fears that their disorders will interfere with the study protocol or cloud the results.

"For drug companies, there's always a balance between trying to limit the study population to those people who will have minimal side effects and have the best treatment response," says Dr. Frank Greenway, an obesity specialist who conducts clinical trials at Pennington Biomedical Research Center in Baton Rouge, La.

Researchers are concerned about treatment response.  A study at Mass General published in 2008 in the Archives of Internal Medicine showed that smokers with depression were 22% more likely to relapse. There is also evidence that certain drugs may worsen depression symptoms.

Pfizer will soon have a better idea of how depression and other psychiatric disorders affect the way people respond to Chantix. In addition to the black box label, the FDA is requiring the company to conduct a large follow-up study on participants with mental health problems.