Teenagers who feel relaxed after their first drag on a cigarette are most likely to become addicted to smoking — a sign that some people’s brains are more susceptible to nicotine, researchers have said.

While peer pressure and other factors may lead young people to try smoking, it is the brain’s response when it is first exposed to nicotine that most determines who becomes addicted, a US study reports.

The study coincides with the introduction today of legislation making it illegal for anyone under 18 to buy tobacco in England, Scotland or Wales. The NHS estimates that about 9 per cent of 11 to 15-year-olds and 26 per cent of 16 to 19-year-olds are smokers.

Researchers at the University of Massachusetts Medical School based their findings on more than 12,000 interviews with 1,246 high school students over four years. Personality factors determined which young people would try smoking, but it was the manner in which the brain reacts to nicotine that was most important in determining if someone would then become hooked, they said.

Experiencing a feeling of relaxation in response to the first cigarette was the strongest predictor of addiction.

Of the 217 teenagers who tried smoking during the study, nearly one-third reported feeling relaxed after inhaling for the first time, and two-thirds of them became addicted to smoking.

The study is published in Pediatrics, the journal of the American Academy of Pediatrics. Joseph DiFranza, who led the research, said: “We know that nicotine can have an immediate impact on the brain, and yet we also know that not every adolescent who tries a cigarette gets hooked.

“It appears that it is an addictive physiology and not an addictive personality that determines who will become dependent.”

The study said one theory suggests that nicotine suppresses pathways in the brain that generate the feeling of craving, which is experienced as relaxation. That in turn creates a craving for nicotine when the drug is absent.

“Cravings represent the desire to repeat a pleasurable drug-mediated experience,” Dr DiFranza, Professor of Family Medicine and Community Health at the university, writes. “These findings underscore our belief that the development of dependence is triggered by the changes in brain chemistry that follow the very first dose of nicotine.”

Among the traits that protected students from becoming addicted were being involved in extracurricular activities, the report said.

Overall, 83 of the 217 participants who tried cigarettes became regular smokers. Other risk factors for addiction among first-time smokers was a depressed mood, a novelty-seeking personality, and familiarity with “Joe Camel,” an animated character used to advertise Camel brand cigarettes.

This particular advertising campaign had ended in 1997, but was still recognised during the study’s interviews between 2002 to 2006. This suggests that “the deleterious effects of cigarette advertising persist long after the exposure”, the researchers said.

The move to raise the legal age for buying tobacco in Britain was welcomed by Ash, the anti-smoking campaign group. Amanda Sandford, a spokeswoman, said that proper enforcement was key to its success.

“It is imperative that retailers fully comply with the law and that enforcement officers make frequent checks on premises selling tobacco,” she said.

Simon Clark, director of the smokers’ lobby group Forest, said measures to discourage children from smoking were welcome, but felt ministers were guilty of sending mixed messages.

“You’re considered old enough to have sex at 16, drive a car and join the army at 17, but the Government doesn't want you to smoke until you’re 18,” he said.

The Department of Health said: “We ran a very targeted age-of-sale awareness campaign, aimed at the primary audience of retailers and 16 to 17-year-old smokers, who represent approximately 0.5 per cent of the population. It is important that we spend taxpayers’ money wisely on targeting the key audiences on this issue.

“The age-of-sale legislation shows our commitment to continue to drive down smoking rates in the UK, as smoking remains the number one cause of ill health and early death.”

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