They increase the risk by around 20 per cent on average, and some risk was still there even five years after stopping.
Women who have taken a hormone contraceptive—and this also includes non-oral devices such as the hormone-intrauterine system (IUS)—for a year or less have a 9 per cent raised risk, and the risk rose to 38 per cent among those who've used hormone contraception for more than 10 years.
Researchers from the University of Copenhagen tracked the health of 1.8 million women for nearly 11 years. During that time, 11,517 women developed breast cancer, and the researchers noted that those who were taking some form of hormonal contraceptive were more likely to do so than those who weren't using the contraceptives. The risk was just as evident among women who were taking the supposedly safer progesterone-only contraceptive.
Overall, the contraceptive was responsible for one extra case of breast cancer for every 7,690 women taking it for at least a year, and there would be one additional case for every 1,500 women taking the pill for five years or longer.
The one piece of good news was that the risk was relative and not absolute. That means that other factors could have played a part in the women getting the cancer—such as diet or pollution—and it couldn't be established that only the pill was responsible