Women who use the contraceptive pill are at a slightly increased risk of getting breast cancer, a new study has confirmed.
But the report's authors said young women should not stop taking it because the hazards were small compared with the health benefits.
Results from one of the largest ever breast cancer studies carried out, and disclosed at a conference in Barcelona at the weekend, show that women's risk of developing the disease rose by just over a quarter if they had ever been on the pill.
That risk, however, varies according to how long they were on the pill, how old they were and how long they live without succumbing to other causes of disease.
The risk for anyone of developing breast cancer under the age of 40 is very low, but it rises with age until it reaches around one in 12 at age 80.
The new study, by scientists from the Institute of Community Medicine in Tromso, Norway, collaborating with researchers in Sweden and France, found no increased risk among women who used oral contraceptives before the age of 20 or before their first full-term pregnancy. Among the 103,027 women, aged between 30 and 49, who joined the study in 1991/92 and were followed to 1999, there were 1,008 cases of breast cancer. Women still on the pill at the end of the follow-up had an increased breast cancer risk of 58%.
The highest risk was among those over the age of 45 who were still on the pill. Their risk was 144% that of never-users.
"It is clear that oral contraceptives increase a woman's risk of developing breast cancer, particularly when they are used in the later period of reproductive life," said Merethe Kumle, the epidemiologist leading the research, at the third European Breast Cancer Conference.
But she cautioned: "It is important to remember that the absolute risk of a woman developing breast cancer under the age of 40 is very low."
The pill has been shown to have health benefits as well as risks. It can have a protective effect against ovarian and endometrial cancer, for instance.
"The total number of deaths from any cause among women who use oral contraceptives is likely to be lower than women who have never used the pill," said Dr Kumle.
