Thousands of women who opt for the mini pill over other forms of contraception may be at higher risk of breast cancer, concerning research today revealed.
Studies have long suggested there may be a link between hormonal birth control and the risk of developing breast tumours.
This is because they often contain the hormones oestrogen and progesterone, which scientists believe can stimulate some cancer cells to grow.
But Swedish scientists, who tracked more than two million women, found the newer progestogen-only pill — often dubbed the mini-pill — carried a higher risk than older alternatives.
Women on the mini-pill, which contains a form of progesterone called desogestrel, were roughly a fifth more likely to develop breast cancer than those not on hormonal contraception.
By comparison, those on the combined pill — which contains oestrogen and progesterone — had a 12 per cent increased risk.
But women on other progesterone-only injections or implants, containing a different type of progesterone to the mini pill, had a ‘lower or no increased risk’.
The researchers said they couldn’t be sure why the mini pill alone put women at greater risk but suggested that further research was necessary into exactly how desogestrel impacts the body.
Studies have long suggested there may be a link between hormonal birth control and the risk of developing breast tumours. This is because they often contain the hormones oestrogen and progesterone, which scientists believe can stimulate some cancer cells to grow
Experts today, who labelled the findings ‘important’, also cautioned it was ‘too early’ to consider changing women’s treatment options yet.
Writing in the journal JAMA Oncology, researchers from Uppsala University said: ‘The finding that desogestrel may increase breast cancer risk more than other types of progesterone is a new finding that, to our knowledge, has not been previously reported.
‘Collectively, results of this study suggest important differences in breast cancer risk between different types of progesterone.
‘These results also suggest that oestrogen may attenuate progestin’s harmful effect.’
Although they were unable to assess the impact of different progesterone daily doses, they added, previous research has suggested that reducing the dose ‘does not proportionally reduce’ breast cancer cells developing.
In the study, the scientists who tracked 2,095,130 teenage girls and women discovered that any use of hormonal contraception was linked to a 24 per cent higher risk of breast cancer.
This meant, that for every 7,752 users, there would be one additional breast cancer case, they hypothesised.
While the combined pill increased the risk slightly by 12 per cent, the mini pill recorded a 21 per cent higher risk.
The proportion of women taking oral contraceptives has fallen by more than two-thirds, from 420,600 in 2012/13 to 126,400 in 2022/23, according to the NHS data
Etonogestrel-containing implants, a substance produced by desogestrel, also had a 22 per cent higher risk.
But there was ‘no significant risk increase’ for other progesterone-only methods including the medroxyprogesterone acetate injection and etonogestrel vaginal ring, the researchers noted, ‘even though they had a large number of users’.
Figures currently suggest around 6 per cent of women aged 16 to 49 use the mini pill in the UK — roughly 3.1 million.
Taken every day, it works by thickening the cervical mucus and thinning the womb lining to stop sperm reaching an egg and attaching itself in the womb.
In some cases, the mini pill can also stop ovulation from occurring.
It is 99.7 per cent effective with perfect use but if used incorrectly — such as missing a pill or experiencing nausea and diarrhoea while on it — around one in ten women (9 per cent) may get pregnant.
Known side effects include nausea, breast tenderness, mood swings and headaches.
Others claim they pile on pounds while taking the mini pill because of increased fluid retention and appetite, yet the NHS says there is no evidence it leads to weight gain.
Decades of research has also failed to provide any conclusive evidence that this supposed side effect is real.
