Other studies have already shown such an association, but the article in the British journal The Lancet innovates by quantifying the risk for each type of treatment and showing that if this over-risk decreases after stopping treatment, persists nevertheless for at least ten years.
The authors reviewed 58 epidemiological studies on the subject, involving more than 100,000 women in total.
Most of these studies are observational, that is to say, they highlight a statistical link but do not show a causal link between the cancer of the women concerned and the treatment followed.
According to their findings, all menopausal hormone therapies (THMs) are associated with increased risk, with the exception of estrogen gels for topical application.
For example, a woman in her 50s who has been on continuous estrogen and progesterone THM for five years has a 8.3% chance of developing breast cancer within 20 years of starting treatment, while risk is only 6.3% for women of the same age who had no treatment.
The proportion would be 7.7% for those who have been treated for the same duration with estrogen and progesterone intermittently (not every day), and 6.8% for those treated with estrogen alone, say the researchers.
The risk also increases with the duration of treatment: "the use of a THM for 10 years leads to an excess risk of breast cancer about twice as high as that associated with a five-year treatment. that using THM for less than a year is a low risk, "says Gillian Reeves of Oxford University, co-author of the study.