A nicotine addiction drives some people to smoke more than others
Smokers who find it hard to cut down or quit may be able to blame their genes, new research suggests.
Scientists identified three genetic mutations that increase the number of cigarettes people smoke a day.
And several genes appear to dictate how likely you are to take up smoking and how easily you can quit.
Three separate studies collected data from 140,000 people, with the results published in the journal Nature Genetics.
A previous study two years ago found a common single-letter change in the genetic code linked to nicotine addiction and lung cancer risk.
This new research confirms this discovery and also pinpoints two more genetic variants that seem to increase cigarette consumption among smokers.
Lung cancer risk
The new single-letter mutations, known as single nucleotide polymorphisms or 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.
However 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.
The University of North Carolina, Oxford University and Icelandic company deCODE were all involved in the research.
Dr Kari Stefansson, researcher and executive chairman of deCODE, stressed that smoking is bad for anyone's health.
"But it is even worse for some, and our discoveries continue to strengthen our ability to identify who those people are and give them a compelling reason to quit."
Amanda Sandford, research manager at ASH, a public health charity, cautioned that any potential benefits from this research were still a long way in the future.
"Smokers who want to quit should not wait for treatment tailored to their genetic make-up.
"There is an abundance of advice and treatments available already to help people stop smoking," she said.