New research has revealed a concerning link between the progestogen-only mini pill and an increased risk of breast cancer, potentially affecting thousands of women who opt for this form of contraception. While studies have long indicated a connection between hormonal birth control and the development of breast tumours, primarily due to hormones like oestrogen and progesterone stimulating cancer cell growth, a recent Swedish study highlights specific risks associated with newer progestogen-only formulations.

Scientists from Uppsala University tracked more than two million women, discovering that the mini-pill, which contains a form of progesterone called desogestrel, carried a higher risk compared to older hormonal alternatives. Women using the mini-pill were found to be roughly 21-22 percent more likely to develop breast cancer than those not using hormonal contraception. In contrast, those on the combined pill, which includes both oestrogen and progesterone, experienced a 12 percent increased risk. Interestingly, women using other progesterone-only methods, such as injections or implants containing different types of progesterone, exhibited a 'lower or no increased risk'.

The researchers could not definitively explain why desogestrel alone posed a greater risk but suggested that further investigation into its precise impact on the body is crucial. Their findings, published in the journal JAMA Oncology, indicated 'important differences in breast cancer risk between different types of progesterone' and proposed that 'oestrogen may attenuate progestin’s harmful effect'. While the study could not assess the impact of varying daily progesterone doses, previous research has hinted that dose reduction does not proportionally decrease the development of breast cancer cells.

Overall, the study observed that any use of hormonal contraception was linked to a 24 percent higher risk of breast cancer. This translates to approximately one additional breast cancer case for every 7,752 users, according to their hypothesis. Specific data showed the mini pill recorded a 21 percent higher risk, while the combined pill increased the risk by 12 percent. Etonogestrel-containing implants, a substance produced by desogestrel, also showed a 22 percent higher risk. However, no significant risk increase was found for other progesterone-only methods, including the medroxyprogesterone acetate injection and etonogestrel vaginal ring, despite their considerable number of users.

In the UK, around 6 percent of women aged 16 to 49, equating to roughly 3.1 million individuals, currently use the mini pill. This contraceptive works by thickening cervical mucus and thinning the womb lining to prevent sperm from reaching an egg and implanting in the womb; in some instances, it can also stop ovulation. It boasts 99.7 percent effectiveness with perfect use, though incorrect usage, such as missing a pill or experiencing nausea and diarrhoea, can lead to a 9 percent pregnancy rate.

Known side effects of the mini pill include nausea, breast tenderness, mood swings, and headaches. While some users report weight gain due to increased fluid retention and appetite, the NHS states there is no evidence to support this claim, and decades of research have failed to provide conclusive proof of this supposed side effect. Furthermore, NHS data indicates a significant decline in the proportion of women taking oral contraceptives in the UK, falling by more than two-thirds from 420,600 in 2012/13 to 126,400 in 2022/23.

Experts have lauded the findings as 'important' but caution that it is 'too early' to consider altering women's current treatment options. They emphasize the necessity for additional research to fully understand the implications of these findings before any clinical recommendations can be made.