Studies sponsored by the pharmaceutical companies selling stop-smoking drugs provide scant evidence of effectiveness in helping smokers achieve long-term abstinence, CNN reported March 12.

Drugs like Pfizer's Chantix and GlaxoSmithKline's Zyban seem to help some smokers quit initially, the success rate drops significantly over time, studies show. "The drugs are approved because they've shown in FDA studies that they're better than placebo," said researcher Edward Levin of the Duke University Medical Center in Raleigh, N.C. "But being better than placebo doesn't take a whole lot, so there really is room for improvement."

For example, about half of smokers using Chantix were able to quit when studied at 12 weeks, but only one in four remained abstinent a year after taking the drug, compared to one in 10 study subjects given a placebo. Among Zyban patients, 14.6 percent were abstinent at one year, compared to 10.3 percent of the placebo group.

"There's nothing out there now that you can take that will make you not smoke," said David Gonzales, co-director of the Smoking Cessation Center at the Oregon Health & Science Center, who has studied both Chantix and Zyban. "That's what people think these drugs do, but they don't do that. The drugs do a reasonable job with suppressing withdrawal, but they don't teach people how not to smoke." 

"The patient may have to go through this quitting process many times," said Martina Flammer, senior medical director of Pfizer's Chantix group. "It is inherent of the nature of nicotine addiction that the patient may actually relapse."

Both drugs also carry the risk of increased suicidal ideation and behavior, the FDA warns. Still, Pfizer sold $888 million worth of Chantix in 2007, and the total market for antismoking therapies is projected to grow to $4.6 billion by 2016.