Teenagers who dabble with tobacco will become life-long addicted smokers if they carry a particular gene, a recent study claims.
About 40% of people with European origins have "high-risk" versions of the genes, which affect the brain's sensitivity to nicotine.
They are in danger of getting hooked if they start smoking before the age of 17, scientists found.
Other variations of the same genes work the opposite way and help prevent addiction.
Researchers studied 2,827 European American smokers and looked for single-letter changes in the genetic code, known as single nucleotide polymorphisms (SNPs), linked to nicotine addiction.
Those who began smoking before the age of 17 and had two copies of the "high-risk" genes were between 1.6 and five times more likely than normal to become an adult heavy smoker.
Study leader Professor Robert Weiss, from the University of Utah, said in the on-line journal PLoS Genetics: "We know that people who begin smoking at a young age are more likely to face severe nicotine dependence later in life. This finding suggests that genetic influences expressed during adolescence contribute to the risk of lifetime addiction severity produced from the early onset of tobacco use."