Smokers who find it hard to cut down or quit may be at the mercy of their genes, new research suggests.
Scientists identified three genetic mutations that increase the number of cigarettes people smoke a day.
Several also appear to be associated with taking up smoking, and one with smoking cessation.
Some of the findings will now be incorporated into risk factor DNA tests developed by the Icelandic company deCODE, which took part in the research.
A previous study two years ago found a common single-letter change in the genetic code linked to nicotine addiction and lung cancer risk.
The new research, which combined data on more than 140,000 individuals, confirmed this discovery and also pinpointed two more genetic variants that seem to increase cigarette consumption among smokers.
Results from the three studies were published today in the journal Nature Genetics.
The new single-letter mutations, known as single nucleotide polymorphisms (SNPs), lie in regions of the DNA molecule containing genes believed to influence nicotine addiction.
In smokers, each copy of the variants was associated with a small increase in smoking consumption equivalent to about half a cigarette a day. But they also conferred a 10% increase in lung cancer risk, raising questions about their effect. It is not clear whether the variants simply drive people to smoke more, or increase susceptibility to cancer as well.