CAN'T quit smoking? The answer may lie in your genes, according to a new study from Dunedin.
Eighteen per cent of New Zealand adults smoke at least once a month and giving up can be difficult.
Most smokers regret ever starting and 60 per cent have tried to quit in the past five years.
Quitline reports 24 per cent of its clients are smokefree at six months.
Now an international team of researchers has pulled together genetic clues from other studies to create a "genetic risk profile" for smoking.
The researchers applied this to a long-term study of 1000 people born in Dunedin in the early 1970s.
Information on participants' smoking behaviour was analysed alongside DNA samples which identified those who matched the smoking risk profile.
Genetic risk score was unrelated to starting smoking, says their research report, published in American Medical Association journal JAMA-Psychiatry.
But for those who did try cigarettes, having a high-risk genetic profile predicted increased likelihood of heavy smoking and nicotine dependence.
The link was strongest for teenagers, says the NZ Science Media Centre.
"Among teens who tried cigarettes, those with a high-risk genetic profile were 24 per cent more likely to become daily smokers by age 15 and 43 per cent more likely to become [20-cigarette] pack-a-day smokers by age 18.
"As adults, those with high-risk genetic profiles were 22 per cent more likely to fail in their attempts at quitting."
Lead researcher Dr Daniel Belsky of North Carolina's Duke University says: "The effects of genetic risk seem to be limited to people who start smoking as teens.
"This suggests there may be something special about nicotine exposure in the adolescent brain, with respect to these genetic variants."
Auckland University tobacco control expert Dr Marewa Glover says the study reinforces the value of policies aimed at preventing young people from starting smoking.
Retail displays of tobacco have been outlawed, "smokefree" signs to discourage smoking have gone up in many parks and the Government is planning to legislate for plain tobacco packaging.