However there was an increased risk for women who used it for more than eight years.

Since its introduction in the early 1960s, more than 300 million women are thought to have used oral contraception. Although several studies have looked at the overall balance of cancer deaths associated with oral contraceptives, none have so far examined the absolute risks or benefits.

Researchers from the University of Aberdeen analysed data spanning a 36 year period from the Royal College of General Practitioners (RCGP) Oral Contraception Study which began in 1968.

The study recruited 46,000 women, with an average age of 29. Approximately half were using oral contraceptives; the other half had never taken it. Every six months their GP provided the study with information on the women's health. In addition, three quarters of the women were 'flagged' at the NHS central registries so that deaths and cancers were notified to the study even if women had left their recruitment GP.

Professor Philip Hannaford and colleagues used the data to calculate the risk of developing any type of cancer and the main gynaecological cancers combined. They also considered the effects of variables such as age, smoking and social class.

Given the length of the study a significant number of GPs had not been able to provide updates throughout the entire study period, for example if a woman moved out of the area of their doctor.

So Professor Hannaford and colleagues from the University of Aberdeen's Department of General Practice and Primary Care calculated the cancer risks using two sets of data. One that related to cancers reported while the women remained registered with their recruiting GP and whose information was more detailed (GP observation dataset). The second main study dataset which was larger, also included cancers notified by the central NHS registries after women had left their recruiting GP.

In both data sets there was no overall increased risk of cancer among pill users. When the GP observation dataset was used, women who had taken the pill at some time during their lives had a 3% reduced risk of developing any cancer. When the larger main dataset was used, the reduction was 12%.

A 12% reduction equates to approximately one fewer case of cancer for every 2,200 women who have used the pill for a year and 3% equates to one fewer case of cancer for every 10,000 women.

In the main dataset women on the pill had statistically significant lower rates of large bowel/rectal, uterine body and ovarian cancer. They also had a 29% reduced risk of developing one of the main gynaecological cancers. The data in the GP data set also showed a reduced risk of uterine and ovarian cancer.

The researchers did sound a note of caution. When the women were examined by how long they had used the pill, women who used it for more than 8 years – less than a quarter of pill users in the study - had a statistically significant increased risk of developing any cancer, in particular cervical and central nervous system cancer.

However the same women were at reduced risk of developing ovarian cancer. Evidence suggests that the protective effect of taking the pill lasts for at least 15 years after stopping.

The average duration of pill use in the study was 44 months.

The authors say that many women, especially those who used the first generation of oral contraceptives many years ago, are likely to find the results reassuring as they conclude:

Professor Hannaford, world expert on contraception, 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.

"Although the results relate to older types of pills, evidence from other studies suggests that currently available pills produce broadly similar effects.

"These findings will probably reassure most pill users living in the UK and their doctors. It should be remembered, however, that the balance of cancer risk may vary around the world, depending on patterns of pill usage and the incidence of different cancers."

Source: University of Aberdeen