Teenagers who report feeling relaxed after they try smoking for the first time are the most likely to become addicted, new research published in Pediatrics has found.

The researchers say their findings are a sign that some people's brains are more susceptible to nicotine than others.

"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," said lead researcher Dr. Joseph DiFranza of the University of Massachusetts Medical School.

"We wanted to know what accounts for the difference in vulnerability among adolescents who pick up that first cigarette."

While peer pressure and other factors may lead young people to try smoking, it is the brain's response to that first dose of nicotine that likely most determines who gets addicted and who will continue to smoke, the researchers conclude.

According to DiFranza's report, experiencing a feeling of relaxation in response to the first cigarette was the strongest predictor of addiction. That may be because nicotine suppresses pathways in the brain that generate cravings, and that suppression is experienced as relaxation.

But in turn, it also creates a craving for nicotine when the drug is absent.

"Cravings represent the desire to repeat a pleasurable drug-mediated experience," DiFranza explains.

The four-year study was based on more than 12,000 interviews with 1,246 Grade 6 students in public schools in six Massachusetts communities. Researchers assessed 46 risk factors in categories such as attitudes and beliefs about smoking, smoking by parents, siblings and peers, family and community involvement, and reactions to inhaling from a cigarette for the first time.

Of the 217 who tried smoking during the study period, nearly one-third reported feeling relaxed after inhaling for the first time, and two-thirds of them became addicted to smoking. Overall, 83 of the 217 participants who tried cigarettes became habitual smokers.

When all factors were considered, an experience of relaxation in response to the first dose of nicotine was the strongest predictor of future addiction.

"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," said DiFranza.

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

Other factors that predicted addiction were familiarity with the cigarette advertising character Joe Camel, a novelty-seeking personality, and a depressed mood.

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