If you're a smoker trying to quit, your genes may determine how effectively a common smoking cessation drug will work, a new study suggests.

The study, by the Toronto-based Centre for Addiction and Mental Health (CAMH), suggests genes play a role in how effectively both nicotine and the smoking-cessation drug buproprion (Zyban) can be metabolized.

"This first study identifies a very common genetic variant (present in anywhere from 25 to 50 per cent of world populations) that appears to affect the outcome of smoking cessation treatment," said lead researcher Rachel Tyndale, section head of pharmacogenetics at CAMH.

The study has been published in the September issue of Biological Psychiatry. Tyndale said the results need be replicated to ensure the study's accuracy.

Researchers focused on one allele of the CYB2B6 enzyme - CYB2B6*6 - because it is carried by 45 per cent of Caucasians, 50 per cent of African Americans and 25 per cent of Asians.

The study, involving 326 participants between April 1999 and October 2001, put them into several groups. One group with the CYB2B6*6 received buproprion to quit smoking, while another group with the allele received placebo.

These two groups were compared to two more: participants with the CYP2B6*1 allele, not known to metabolize buproprion, were given the smoking cessation drug and placebo as well.

After 10 weeks of treatment, the participants were evaluated for one week and again at six months.

Researchers found buproprion worked more effectively than placebo among smokers who had the CYB2B6*6 allele. Smokers taking buproprion reported greater abstinence rates, with 32.5 per cent saying they had resisted the urge to smoke versus 14.4 per cent taking placebo.

Similarly, at six months, 31.2 per cent of buproprion users with the CYB2B6*6 allele were still not smoking versus 12.9 per cent taking placebo.

Conversely, for those smokers with the CYP2B6*1 allele, buprioprion was no more effective than placebo at preventing smoking; 31 per cent of smokers taking the drug and 31.6 per cent on placebo reported abstaining from smoking.