Smoking just one cigarette can cause tobacco addiction in some people, according to a New Zealand study published on Wednesday.
Medical researchers asked 96,000 youngsters aged 14-15 to fill in questionnaires about whether they smoked and whether they felt the need to continue smoking.
Those who smoked frequently replied, as expected, that they felt the urge to continue smoking.
But what surprised the investigators was the number of infrequent smokers who also reported a craving.
Forty-six percent of those who smoked less than one cigarette per month said they had diminished control over the urge to smoke.
Even more remarkable was that among the teenagers who said they found it hard to repress an urge to smoke, 10 percent had the impulse within two days of smoking their first cigarette and 25 percent within one month of that event.
To calibrate any dependence on tobacco, the questionnaire, which was issued via schools between 2002 and 2004, included such questions as "Do you ever have strong cravings to smoke?", "Do you smoke now because it is really hard to quit?" and "Did you find it hard to concentrate because you couldn't smoke?"
The responses, say the authors, confirm previous research showing addiction rises as more cigarettes are smoked, and sets in soon after the first puffs.
Indeed, "these data suggest that smoking one cigarette in total can prompt a loss of autonomy," says their report, published on Wednesday in the Elsevier journal Addictive Behaviors.
In 2004, a study among 1,300 13-year-olds in Canada's Quebec province found evidence that those with variants of a key gene that mops up nicotine in the liver were far likelier to get hooked on smoking.
Those with two variants in the CYP2A6 gene were nearly three times more likely to become addicted to smoking compared to those with the regular form of the gene.
The genetic variants decrease concentrations of nicotine-degrading enzymes in the liver. As a result, nicotine -- a "reward" chemical in the brain -- is processed more slowly, and the sense of pleasure from smoking lasts longer.
Another variant implicated in smoking addiction is a form of a gene called DRD 32806, which controls dopamine, a brain chemical that is critical for the sensation of reward.
Smoking kills around five million people a year, with female smokers and smokers in developing countries the most vulnerable group.