Smokers who use pills to try to kick the habit should keep trying, even if the drugs do not work at first, a U.S. researcher advised on Saturday.

It can take several weeks for many people to fully wean themselves off tobacco, the researcher at Oregon Health & Science University said in a statement.

"Our recent analysis shows smokers and clinicians should not be discouraged when total abstinence is not achieved in the first weeks of treatment with smoking cessation medications," said David Gonzales, director of the university's smoking cessation center.

Gonzales presented his work at a meeting of the Society for Research on Nicotine and Tobacco in Austin, Texas.

Gonzales and colleagues have been studying how well the drugs Chantix or Zyban work, and published one study last year. They analyzed this and another study to see if it took some people longer to quit than others when they used the drugs.

Varenicline, marketed as Chantix by Pfizer and bupropion, marketed as Zyban or Wellbutrin by GlaxoSmithKline, can both help smokers quit and are licensed for that use. Zyban is an antidepressant and Chantix is designed to block nicotine in the brain.

Gonzales and colleagues evaluated Chantix versus Zyban in more than 1,000 smokers from June 2003 to April 2005 and found Chantix worked better to help people quit.

But no one looked at what would happen if those who did not succeed kept trying.

"If smokers on medication don't quit in the first week or two following their target quit date, clinicians often will instruct their patients to discontinue their medication. Clinicians consider this a failed attempt," said Gonzales, who has had contracts with Pfizer, Glaxo and several other drug companies.

Researchers found that 24 percent of those taking Chantix were able to quit right away, versus 18 percent taking Zyban and just 10 percent given a placebo.

But an additional 20 percent of those taking Chantix and 11 percent taking Zyban were able to quit if they kept trying through three months.

Most experts agree that tobacco is as addictive as heroin and cocaine, and people often take many tries before they are able to stop.