WASHINGTON (MCT) — A new anti-smoking drug improves the odds of success threefold for people who want to quit, an independent research group reported Tuesday.

The drug, called Chantix by its maker, Pfizer Inc., outperformed the antidepressants that helped some quitters in clinical trials that the British-based Cochrane Collaborative reviewed.

In the trials, the antidepressants outperformed the placebos used to measure Chantix’s effectiveness by 2-to-1, while Chantix showed a 3-to-1 advantage over the control group.

A third drug-based approach, nicotine replacement therapy, at best only doubled the odds of quitting successfully, according to a 2004 review by Cochrane.

Cochrane’s panels of volunteer physicians and health researchers appraise the quality of all known studies and the findings of the most solid ones. Health professionals and insurers study Cochrane’s evaluations, published online as the Cochrane Library and available by subscription, for the efficiency they add to health-care spending.

Cochrane’s findings on Chantix reinforce the Food and Drug Administration’s decision last May to approve the drug on an expedited basis “because of its significant potential benefit to public health.” The findings also bolster the reputability of Chantix’s six clinical trials, all sponsored by Pfizer.

While Cochrane’s reviewers deem those findings solid, they call for more independent research on the grounds that industry-funded trials “are more likely to have outcomes favorable to the product sponsor.” None of the panelists reported receiving support of any kind from Pfizer.

Chantix is based on the theory that people become addicted to smoking because nicotine stimulates the nervous-system receptors that release the feel-good hormone dopamine.

The chemical on which Chantix is based, varenicline, stimulates those receptors to release a little dopamine, thus reducing nicotine-withdrawal symptoms. It also blocks the receptors from absorbing nicotine, making smoking less satisfying.

In clinical tests, roughly half of the 5,000 participants each took 1 milligram of varenicline twice daily for 12 weeks. The rest took either a placebo or bupropion, a prescription antidepressant sold as Wellbutrin or Zyban that helps some people quit smoking. All participants received smoking-cessation counseling and supportive post-treatment phone calls for the remainder of a year.

By year’s end, roughly four out of five participants were smoking again. But among those who managed to quit, varenicline proved one and a half times as effective as bupropion, which was twice as effective as the placebo.

Chantix costs $100 to $140 a month, and insurance generally doesn’t cover it. The Cochrane report notes that there’s no clinical evidence of how likely Chantix users are to relapse or how well the drug will work for those who do.

John Polito of Mount Pleasant, S.C., a leading advocate of quitting cold turkey, said smoking-cessation studies financed by drugmakers routinely overstated their benefits. In addition, while Chantix buyers will be offered free Web site support for their anti-smoking efforts, Polito doubted that this will be as effective as the 10-minute personal counseling sessions that test participants received.

Varenicline is derived from the naturally occurring alkaloid compound cytisine, which was developed in Bulgaria in the 1960s to help smokers quit. Cytisine is extracted from a bush known as goldenrain, which grows in southern Europe, and is sold via the Internet under the trade name Tabex.

A second Cochrane anti-smoking study, also released Tuesday, found marked differences in effectiveness among antidepressants. Zyban and Wellbutrin doubled a person’s chances of giving up smoking, as did Pamelor and Aventyl, commercial names for the drug nortriptyline. However, a third class of antidepressants did no good, Cochrane’s evaluators found, for reasons they couldn’t explain. It includes Prozac and other drugs described as selective serotonin reuptake inhibitors.

About 45 million Americans smoke. Another 46 million have quit.