The researchers analysed information on smoking habits and symptoms of nicotine dependence gathered from almost 1,200 grade 7 students in 10 schools.

Blood samples were taken to identify genetic profiles, and 228 students who smoked but were not yet addicted were subsequently monitored for around two years.

'Inactive' Gene Linked to Addiction

During this period, 67 students became addicted to nicotine. Dependency was significantly more likely among those students with the "inactive" gene variants of the CYP2A6 gene (CYP2A6*2 or CYP2A6*4).

They were almost three times as likely to become addicted to tobacco as those with the normal variants of the gene. Those with partially inactive variants of the CYP2A6 gene were not at increased risk of becoming addicted.

Students with the normal gene smoked an average of 29 cigarettes a week. Those with the partially inactive variant smoked 17. But those with the inactive variant, which slowed down nicotine clearance the most, smoked around 12 cigarettes a week.

Lighter Smokers Get Hooked

The authors suggest that slow clearance prolongs the exposure of the brain to nicotine and is likely to make it more intense. This may boost the effects of the physiological processes that lead to dependence/addiction, and result in the requirement for fewer cigarettes to achieve it.

It also means that while young smokers with the slowest variant are likely to smoke less, they are also likely to get hooked more easily, say the authors. Previous research has shown that it is much harder for those who become chemically dependent on nicotine to quit smoking.

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