Smokers who take Pfizer's pill Chantix to help them quit do not have a higher risk of being hospitalized for psychiatric events such as depression, compared with nicotine patches, according to studies reported on Monday.
Users of Pfizer's non-nicotine pill have reported agitation, depression and suicidal thoughts, and, in clinical trials, the pill was linked to nightmares.
Psychiatric symptoms have occurred in people without a history of mental illness and have worsened in people who already had mental illness, the FDA has said.
Although investors initially had high hopes for the drug, called Champix in Europe, reports of psychiatric effects have hampered sales growth.
The Food and Drug Administration released its review of two studies which compared Chantix to standard nicotine replacement therapy such as the Nicoderm nicotine patch from GlaxoSmithKline.
However, the FDA said the studies had limitations, and it would keep a restrictive "black box" warning on the smoking-cessation aid, advising about psychiatric side effects.
The drug has also been linked to a small increase in heart risk for those who already have heart problems, the FDA said in June.
"Overall, FDA has determined that the current warnings in the Chantix drug label, based on postmarketing surveillance reports, remain appropriate," the agency said in a statement.
Annual sales are now about $800 million, making the pill a moderate-sized product for the world's biggest drugmaker.
The FDA said about 9.8 million people got Chantix prescriptions from U.S. retail pharmacies from the time the drug was approved in May 2006 through July of this year.
In one of the studies reported on Monday, the Department of Veterans Affairs compared 14,131 veterans using Chantix with an equal number of those using nicotine patches to see which group was hospitalized more frequently for psychiatric treatment during one year.
In the second study, the Department of Defense compared almost 20,000 Chantix users to about 16,000 people using nicotine patches for 30 days after each began treatment to stop smoking.
The FDA said the sample sizes in both studies were too small to analyze rare events. They also did not count psychiatric events that did not lead to hospitalization.
Pfizer is currently conducting its own large-scale trial to look at the risks of psychiatric events with Chantix, with results expected in 2017, the FDA said.
The company's shares were up 0.5 percent to $19.15 on the New York Stock Exchange, versus a 0.2 percent rise in the S&P Pharmaceuticals Sub-Industry Index.
(Reporting by Anna Yukhananov in Washington; editing by Derek Caney)