Some people who find it hard to give up smoking may have a good excuse - it's down to their genetic make-up.
Some find it harder to quit than others
Scientists have found that people who carry a version of one particular gene may find it harder to give up their habit.
However, there may be an upside. It seems that the same variant may protect people from developing lung disease.
The gene in question - dubbed CYP2A6 - plays a role in processing nicotine in the body.
However, there are at least three versions of the gene. One, called del, is less active than the others.
People with the gene could be warned about it and it might help dissuade some from starting to smoke.
A team of researchers from Keio University in Tokyo took DNA samples from 203 current or ex-smokers suspected of having respiratory disease, and compared them with samples from 123 healthy non-smokers.
They found that current smokers were more likely to carry the del version of the gene than people who had managed to quit.
This may be due to their inability to break down nicotine in their bloodstream. Hence, levels are constantly high, and the process of giving up - which would see nicotine levels fall - would be more of a shock to the system than in other smokers more easily able to regulate nicotine levels in their blood.
However, smokers who did carry the del variant were less likely to consume large amounts of tobacco - possibly because they need less to keep nicotine levels topped up.
Tests on participants' lungs also seemed to indicate that carrying the del gene seemed to help to protect against the respiratory disease pulmonary emphysema.
Why this should be is not clear, but the researchers believe the gene may play a role in breaking down, or blocking production of other substances that may trigger inflammation, and thus disease.
Writing in the journal Thorax, the researchers say: "These findings suggest that determination of the genotype will be useful in efficiently withdrawing patients from nicotine dependence in smoking cessation protocols with nicotine containing materials."
Lead researcher Dr Hidetoshi Nakamura told BBC News Online that the del version of the gene was particularly common among Asian people - it is carried by about 30% of Japanese.
Deborah Arnott, director of the anti-smoking ASH, said: "This research could be useful as people with the gene could be warned about it and it might help dissuade some from starting to smoke.
"It could also enable the tailoring of smoking cessation treatment which may need to be different for people with this genetic history."
However, she added: "While the gene may provide some protection against emphysema, people need to realise this is only one of the many diseases caused by smoking which kills 50% of those who smoke long term, not just through emphysema but also through lung cancer, coronary heart disease and stroke."