An observational study from Canada has shown that a decline in the use of HRT over the past decade was followed by a reduction in the incidence of breast cancer.

Data on HRT prescriptions dispensed for women aged 50-69, during the years 2001-2006, was collated from a national registry. Information about current HRT use was obtained by telephone from a sample of 1,200 women in this age group already enrolled in the National Population Health Survey. Subjects were asked ‘in the past month, did you take hormones for menopause or ageing symptoms?' Those who answered affirmatively were asked about the type of formulation used.

Age-standardised incidence rates for breast cancer were obtained from the Canadian Cancer Registry.

The number of HRT prescriptions dispensed declined over time from 11.6 to 8.5 and then to 5.4 million in the years 2002, 2003 and 2006 respectively. This was reflected in the longitudinal health survey findings. In 2002, 12.7% of women surveyed said they were current users of combined HRT. By 2004 this figure had dropped to 4.9%.

Breast cancer incidence rates in 50-69 year olds fell by 9.6% over the same period from 296.3 per 100,000 women in 2002 to 268 per 100,000 in 2004. Mammography rates were stable (72%) over the same period.

Studies conducted in other countries have shown similar findings. It would seem logical that prolonging a woman's exposure to sex hormones would increase risk (breast cancer is slightly more common in women with early menarch and late menopause). However, the data are observations of association only, and are not controlled for other risk factors for breast cancer such as parity and alcohol intake.

The absolute risk of breast cancer to an individual user of HRT is small (and smaller still in oestrogen-only formulation users). The potential benefits need to be weighed up against the risks for each individual patient.

Dr Chris Barclay