This slight increase in the risk of breast cancer must, however, be put into perspective in the face of the benefits derived from contraception, including the protection provided against other types of cancer, the researchers stress.

The increased risk of breast cancer was already well known for contraceptive methods combining progestin and estrogen.

But while the use of progestin-only methods has been on the rise for years, few studies have so far looked at their own effect on breast cancer risk.

This is actually similar to estrogen-progestin methods, concludes this study published in the journal PLOS medicine.

According to this work, women using hormonal contraception have an increased risk of about 20% to 30% of developing breast cancer -- regardless of the mode of delivery (pill, IUD, implant or injection), or the formula used (estrogen-progestin or progestogen alone).

This rate is similar to what previous studies, including a large study, had estimated as early as 1996.

To give a better idea of what this represents, the researchers calculated the number of additional cases of breast cancer caused, knowing that the risks of developing this disease increase with age.

In the case of hormonal contraception taken for five years between the ages of 16 and 20, the number of women developing breast cancer in 100,000 will be eight. Taken between the ages of 35 and 39, this represents 265 additional cases of breast cancer per 100,000 women.

Transient risk

"Nobody wants to hear that something they take is going to increase their risk of breast cancer," acknowledged Gillian Reeves, a professor at Oxford University and co-author of the study. But it is a "very small risk in terms of absolute risk," she told a news conference.

This risk must also be considered in the light of the benefits provided by hormonal contraception, "not only in terms of pregnancy control, but also because oral contraceptives provide quite significant and long-term protection against other cancers in women, such as endometrial ovarian cancer," said Gillian Reeves.

In addition, the study confirms, as others before it had done, that the increased risk of breast cancer linked to hormonal contraception is transient: it declines in the following years when contraception is stopped.

These results "are reassuring because the effect is modest," said Stephen Duffy, a professor at Queen Mary University of London, who was not involved in this work.

This study was carried out using data from just under 10,000 women under the age of 50 who developed breast cancer between 1996 and 2017 in the United Kingdom, where progestin-only contraceptives are now as common as those combining progestin and estrogen.

Progestin-only contraceptives are recommended for breastfeeding women, or for those with contraindications to estrogen-progestogen pills, such as cardiovascular disease risks, or if smoking after age 35.

Among the "multiple factors" explaining the increase in their use, it may be that "women today take contraceptives later in life, and therefore that they have more of these conditions naturally, said Gillian Reeves.

© 2023 AFP