Progestin birth control has risk of cancer

Many women are taking progestin-only contraceptives, whose association with breast cancer risk isn’t well-known.

A new study says that the risk associated with progestin-only pills is broadly in line with the risk already known to be associated with contraceptives that combine estrogen and progestin. Progestin is a synthetic form of the hormone progesterone; estrogen also is a hormone.

The results are “not surprising,” said Gill Reeves, a co-author of the study and professor of statistical epidemiology at the University of Oxford, in an interview. In fact, “it may be reassuring to know that these newer contraceptives that women are using in increasing numbers do not have any untoward effects that might be unexpected. They do seem to behave pretty much as the traditional contraceptives.”

The peer-reviewed analysis, conducted by researchers at the University of Oxford and published in the PLOS Medicine journal Tuesday, also provided more evidence that the use of all kinds of hormonal birth control is associated with a slight increase in the risk of breast cancer.

Given the small incidence of breast cancer at the ages at which contraceptives are commonly used, however, researchers do not suggest women stop taking hormonal birth control in light of any risk — which they say is only small.

The study’s authors compared data from 9,498 women under the age of 50 who had been diagnosed with invasive breast cancer with more than 18,000 closely matched control subjects who hadn’t been diagnosed with cancer.

The researchers found that 44 percent of the women in the group with breast cancer had taken a hormonal contraceptive which was prescribed around three years before their cancer diagnosis, on average.

That was higher than the rate among women who hadn’t received a breast cancer diagnosis, 39 percent of whom had received hormonal birth control. The findings suggested that hormonal birth control is associated with a relative increase of 20 to 30 percent in breast cancer risk, the study authors said.

In a separate part of the research, which combined their findings with previous studies, the researchers found that, in practice, any increased cancer risk was slight given the small existing risk.

According to the results, women taking hormonal birth control between the ages of 16 and 20 could see a further eight cases of breast cancer per 100,000 users, Reeves said, “which, when you put in those terms, sounds considerably different to saying it’s a 25 percent increase.”

For women between ages 35 and 39, this increased to 265 cases per 100,000 for hormonal contraceptive users — but the increase is in line with the rise in breast cancer cases as women age, Reeves noted.

The study found “no material difference” in the cancer risk associated between various forms of contraceptives, such as combined or progestin-only pills, and injections and IUDs using progestin.