The study, published in The Lancet, used data from 108,647 women who had developed breast cancer to investigate the impact of taking menopausal hormone therapy (otherwise known as hormone replacement therapy or HRT) on breast cancer risk. Half of these women had used HRT, while the other half had never used it. 

The average age that the HRT treatment was started was 50. If women were taking HRT at the time of their breast cancer diagnosis, the average amount of time they had been taking it was 10 years. If they had stopped, they had taken the HRT treatment for, on average, 7 years. 

The study found breast cancer risk increased the longer women took HRT. The exception was women receiving topical vaginal oestrogens which led to no increase in their risk. This increase lasted several years after stopping HRT, with a higher risk for women who had been taking HRT for longer. However, there was little additional risk of women developing breast cancer if they had been taking HRT for a year or less before discontinuing.

The type of HRT women were taking also had an impact on their risk, with the women taking a combined HRT (containing oestrogen and progestogen) at higher risk of developing breast cancer that those who were taking oestrogen-only. The risk was greatest for developing oestrogen receptor positive breast cancer, and the age at which the HRT treatment was started had little impact on the risk.

Overall, the paper concluded that for women of average weight starting HRT at the age of 50 and taking it for five years: 

  • Taking combined HRT where progestogen is included every day could lead to an additional one breast cancer case in every 50 users between ages of 50-69

  • Taking combined HRT where progestogen is given intermittently could lead to an additional one breast cancer case in 70 users between ages of 50-69

  • Taking an oestrogen-only preparation could lead to an additional one breast cancer case in every 200 users between ages of 50-69

Further research is also now needed to fully understand how and why HRT increases breast cancer risk, including the mechanism behind the greater effect of combined HRT on breast cancer risk.

Baroness Delyth Morgan, Chief Executive at Breast Cancer Care and Breast Cancer Now, said:

While we’ve long known that HRT increases the risk of breast cancer, this major study suggests it may take much longer than previously thought for this risk to return to normal after women stop taking it, which could be important new information to consider.

Taking HRT is a really personal decision, and it’s vital that everyone fully understands the benefits and risks, discusses them with their GP and is supported to make the decision that’s right for them. These robust findings also tell us more about the long-term impact of different types of HRT on breast cancer risk, and, rather than causing concern, we hope they will help anyone considering HRT treatment to make an even more informed decision.

On balance, many women will feel HRT to be a necessity, as it can be really effective in helping them control debilitating menopausal symptoms such as hot flushes. In order to minimise the risk of breast cancer, it is normally recommended that the lowest effective dose is used for the shortest possible time. 

Weighing up the benefits and risks of taking HRT can be incredibly difficult, and we’d encourage anyone concerned about HRT and their breast cancer risk to speak to their GP or call our free Helpline on 0808 800 6000.