A new stop-smoking drug appears to work better than drugs currently on the market, but it's likely that no pill will ever be 100 per cent effective, scientists say.

Three studies published this week suggest that varenicline, a new smoking cessation drug developed by Pfizer, is more effective in the short term than Zyban, manufactured by rival GlaxoSmithKline.

However, an editorial accompanying the studies cautions that varenicline is no miracle drug and most people who took it during the studies did not stop smoking.

Pfizer funded all three of the studies, published in Wednesday's issue of the Journal of the American Medical Association.

Varenicline stimulates nicotine receptors enough to release dopamine in the brain, reducing the craving for cigarettes and the withdrawal effects, but not so much that it is itself addictive. 

Third type of anti-smoking aid

Many pharmaceutical stop-smoking aids, such as the patch, contain nicotine and gradually reduce the dose over time to make withdrawal from cigarettes more gradual.

Another drug, bupropion, is an antidepressant that reduces the severity of nicotine cravings and withdrawal symptoms. It is marketed as Zyban for this purpose and as Wellbutrin as an antidepressant.

Two of the studies were conducted in the U.S. and compared the effectiveness of varenicline in helping patients quit smoking in comparison with bupropion and a placebo.

Patients in these trials were treated for 12 weeks and their smoking was monitored for a year. During the last four weeks of their treatment, about 44 per cent of patients taking varenicline were still abstaining from smoking, compared to 30 per cent of those taking bupropion and 18 per cent taking the placebo.

After a year, about 22 per cent of those taking varenicline still had not resumed smoking, compared to eight per cent of those taking the placebo and 16 per cent of those taking bupropion. The authors said varenicline was not significantly more effective than bupropion after one year.

The third study was conducted to determine whether taking varenicline over a longer period of time could prevent people who had stopped smoking from starting again.

Most still smoking a year later

Patients were given varenicline for 12 weeks and about 64 per cent of them didn't smoke during the last week. Some of those who stopped smoking were then treated for another 12 weeks, with either varenicline or a placebo.

At the end of those 12 weeks, about 70 per cent of those taking varenicline has not smoked, compared to about 50 per cent of those taking the placebo. After a year, about 44 per cent of those taking the drug had not smoked, compared to 37 per cent of those taking the sugar pill.

The authors point out that, as in all studies on smoking cessation lasting a year or more, more than 50 per cent of people who began treatment started smoking again.

"Cleary, quitting smoking, even with pharmacological and behavioural assistance, is extremely difficult," write three researchers at the University of Tennessee Health Science Center in Memphis, in an editorial accompanying the studies. "Patients currently cannot and probably never will simply be able to 'take a pill' that will make them stop smoking."