Genetics may be responsible for smokers finding it difficult to quit, scientists have said.
Three genetic mutations have been linked to increasing the number of cigarettes a person smokes each day, while several were associated with starting the habit and one with stopping.
The study used data from more than 140,000 people, and confirmed previous research which found nicotine addiction and the risk of lung cancer was linked to a common single-letter change in genetic code.
Two more genetic variants associated with cigarette consumption among smokers were discovered during the latest study, with the results published in the journal Nature Genetics.
Scientists were not certain whether the variants increased an individual’s susceptibility to cancer, or just encouraged them to smoke more.
Icelandic company deCODE, which took part in the research, said it will incorporate the findings into risk factor DNA tests.
Dr Kari Stefansson, executive chairman of deCODE and one of the researchers from 23 institutions spread across 12 countries, said: “What is clear is that these variants - which are all near genes that encode nicotine metabolising enzymes and receptors - are giving us a solid starting point for finding answers to advance personal and public health.”