CHICAGO (Reuters, Oct 1) - Teenagers who feel relaxed after their first drags on a cigarette are most likely to become addicted to smoking, a sign that some people's brains are more susceptible to nicotine, researchers said on Monday.

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

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, according to DiFranza's report published in Pediatrics, the journal of the American Academy of Pediatrics.

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

The study said one theory suggests 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," DiFranza wrote.

Over the four years of the study between 2002 and 2006, 11 interviews were conducted with each of 1,000 teenagers attending public schools in six Massachusetts communities.

Of the 217 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.

Overall, 83 of the 217 participants who tried cigarettes became smokers.

Other risk factors for addiction among first-time smokers was a depressed mood, a novelty-seeking personality, and familiarity with "Joe Camel," the animated character used to advertise Reynolds American Inc's Camel brand, the study said.

"The Joe Camel campaign was discontinued in August of 1997 as our subjects entered 2nd grade (around age 7), suggesting that the deleterious effects of cigarette advertising persist long after the exposure," the study said.

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