One of the largest studies ever undertaken claims that women who take the contraceptive pill over a short period of time may reduce their chances of getting cancer.
But researchers at the University of Aberdeen found those who take it for more than eight years may actually increase their risk of suffering from the disease.
The study in the British Medical Journal (BMJ) found that those who take the contraceptive for shorter periods suffer from lower rates of bowel and rectal cancer, uterine cancer and ovarian cancer.
But taking it over an extended timescale can make women more susceptible to cervical and central nervous system cancers, it added.
The study looked at 46,000 women over the last 36 years, and analysed data from GPs and central NHS registers.
An estimated three million women use the pill each year in the UK, and 100 million around the world.
More than 300 million women are thought to have used the pill since its launch in 1961.
Professor Philip Hannaford, from the University of Aberdeen, said: "These results show that in this UK cohort the contraceptive pill was not associated with an overall increased risk of any cancer, indeed it may produce an important net public health gain.
"These findings will probably reassure most pill users living in the UK and their doctors."