WASHINGTON — A tablet shown to help more than one in five smokers quit on Thursday joined the limited number of effective stop-smoking drugs approved by federal regulators.

When varenicline goes on sale later this year, it will become the first new prescription drug for smoking cessation approved by the Food and Drug Administration in nearly a decade and only the second stop-smoking drug that is nicotine-free, according to Pfizer Inc.

The New York company plans to market the twice-daily tablet, intended for adults only, as Chantix.

"It's a welcome new addition. It's like with cancer or heart disease or high blood pressure or diabetes: The more effective treatments you have, the better off patients are," said Dr. Steven Schroeder, a professor of medicine at the University of California, San Francisco.

Varenicline works in two ways, by cutting the pleasure of smoking and reducing the withdrawal symptoms that lead smokers to light up over and over again.

Most other stop-smoking drugs are various nicotine-replacement therapies, sold by prescription and over the counter in gum, patch, lozenge, nasal spray or inhaler form. In 1997, the FDA approved bupropion, an antidepressant already sold as Wellbutrin but then rebranded as Zyban.

Several studies conducted in Europe on about 2,000 smokers and presented in November at an American Heart Association conference showed that a year after initial treatment with varenicline, abstinence rates were 22 percent, versus 16 percent among those given Zyban. Just 8 percent of those given dummy medicines had stopped after a year.

The approved course of Chantix treatment is 12 weeks, a period that can be doubled in patients who successfully quit to increase the likelihood they will remain smoke-free, the FDA said.

Other clinical trials have shown that the drug's effect is more pronounced in the short-term: 44 percent of longtime, pack-a-day smokers quit following a 12-week course of treatment with Chantix, compared with 30 percent of Zyban patients who quit, according to Pfizer. However, smoking cessation experts said the longer-term data are more applicable, given the difficulty of quitting the habit for good. Even Pfizer acknowledged it can take smokers 10 attempts.

"It's not going to be a revolution, it's going to be a substantial step forward," Thomas Glynn, director of cancer science and trends at the American Cancer Society, said of varenicline. Glynn added that the greatest value will be for smokers who have tried Zyban or nicotine-replacement therapy but failed to quit.

"My bet is that it will work as well as they do and, from the look of things, a little bit better," he said. The FDA does not recommend that Chantix be used with any other stop-smoking drug.