A new study has found that hormone replacement therapy soon after menopause may increase breast cancer risk.

The new analysis of the worldwide evidence (bringing together data from 58 studies, including more than 100,000 women with breast cancer) demonstrates a link between different forms of menopausal hormone therapy and breast cancer incidence, and find that some risk persists for many years.

For women of average weight in Western countries, five years of menopausal hormone therapy (MHT), starting at age 50 years, appears to increase breast cancer incidence from age 50 to 69 years by about one additional case in every 50 users of oestrogen plus daily progestogen MHT, one in every 70 users of oestrogen plus intermittent progestogen MHT, and one in every 200 users of oestrogen-only MHT.

It also finds that after ceasing MHT, some excess risk persists for more than 10 years, but the size of this risk depended on the duration of previous use.

If a woman had used MHT for less than a year, however, there was little excess risk thereafter.

Previous studies had shown that current and recent users of MHT were at an increased risk of breast cancer, but insufficient information was available about the effects of different types of MHT or about long-term risks after MHT use had ceased.

The authors estimate that for women with five years use of the three main types of MHT, starting at age 50, the 20-year breast cancer risks from ages 50 to 69 inclusive would increase from 6.3 per 100 in never-users to:

  • 8.3 per 100 in users of oestrogen plus daily progestagen (i.e, 83 in every 1,000 users would develop breast cancer) – an absolute increase of 2 per 100 users (one in every 50 users)
  • 7.7 per 100 in users of oestrogen plus intermittent progestagen (i.e, 77 in every 1,000) – an absolute increase of 1.4 per 100 users (one in every 70 users)
  • 6.8 per 100 in users of oestrogen-only (i.e, 68 in every 1,000 users) – an absolute increase of 0.5 per 100 users (one in every 200 users)

“Use of menopausal hormone therapy for 10 years results in about twice the excess breast cancer risk associated with 5 years of use,” says Professor Gillian Reeves from Oxford University.

“But, there appears to be little risk from use of menopausal hormone therapy for less than one year, or from topical use of vaginal oestrogens that are applied locally as creams or pessaries and are not intended to reach the bloodstream.”

The full report is accessible here.