HRT and cancer risk

This is a more complicated question than it may sound. To help you sort it all out, we've organised our answer into the following sections

• What HRT is
• HRT and womb cancer risk
• HRT and ovarian cancer risk

• Overview of HRT and breast cancer risk
• What we know about breast cancer risk and HRT
• HRT if you already have breast cancer

HRT (hormone replacement therapy) involves taking female sex hormones to replace hormones that your ovaries no longer produce after menopause. HRT can reduce the symptoms of menopause that many women have. It can also help to lower the risk of osteoporosis (thinning bones as women get older). Doctors used to think it helped to protect against heart disease, but recent studies seem to show that it can increase the risk of heart disease.

There are two types of HRT. One type contains only oestrogen and is called oestrogen only HRT. The other contains oestrogen and progesterone – both of the female sex hormones – and is called combined HRT.

Unless you have had a hysterectomy, oestrogen only HRT can increase your risk of cancer of the womb lining (endometrial cancer). Your doctor won't give you oestrogen only HRT unless you have had a hysterectomy. Combined HRT does not increase the risk of endometrial cancer.

Taking HRT increases the risk of developing ovarian cancer. Several small studies have shown this, and in April 2007 the results of a big trial of almost a million women showed that taking any type of HRT slightly increases the risk. The longer HRT is taken, the more the risk increases. But when the HRT is stopped, the risk goes back down to normal over a few years.

The Million Women study showed that over 5 years there is one extra case of ovarian cancer in every 2500 women who take HRT, so the risk is only slightly increased. But, HRT also increases the risk of dying of ovarian cancer. The Million Women study showed that over 5 years, for every 3,300 women who take HRT, one extra woman will die due to ovarian cancer when compared to a group of women who have never taken HRT. If you are worried about HRT and ovarian cancer, you should speak to your GP, who can discuss the risks and benefits in your individual situation.

The cancer most researched in relation to HRT is breast cancer. Research has looked into both oestrogen only and combined HRT. It has shown that HRT does increase breast cancer risk. And combined HRT increases breast cancer risk more than the oestrogen only type. The longer you take HRT, the more your breast cancer risk increases.

The good news for women already taking HRT is that your risk seems to go back to normal within 5 years of stopping taking it. If you are worried about taking HRT, you can talk to your doctor about the benefits and risks in your individual situation.

Results of a very large Cancer Research UK study were published in August 2003. This study recruited over a million women. It showed, for the first time, that combined HRT was more likely to cause breast cancer than oestrogen only HRT. Before this study was published, experts thought that oestrogen only HRT was more likely to cause breast cancer. In women aged between 50 and 65 who do not take HRT, 32 per 1000 will develop breast cancer.

At the time of publication, the Million Women study had followed the women for up to 4 years. The researchers estimated that there are likely to be about 19 extra breast cancers diagnosed in every 1,000 women taking combined oestrogen and progesterone HRT for 10 years. This compares to their estimate of about 5 extra breast cancers for every 1,000 women who take oestrogen only HRT for 10 years.

Another study was published in the British Medical Journal in August 2005. The authors of this study think that breast cancer risk may be a little lower than in the Million Women Study. They have worked out a chart of risk by age so that GPs have some information to guide them when discussing this tricky subject with their patients. In women between the ages of 40 and 79 not taking HRT, 72 out of a thousand will develop breast cancer. Researchers estimate that for every 1,000 women taking HRT for 10 years in that age group there will be about 12 extra cancers. So for women taking combined HRT for 10 years, 84 per thousand will develop breast cancer.

In March 2008, the Women's Health Initiative trial in America reported results of a trial which supports the evidence that combined HRT increases the risk of breast cancer. The study included almost 16,000 women aged between 50 and 79. Half of the women took combined HRT and half took a placebo. In 2002, the women had taken the HRT or placebo for an average of 5.6 years. The researchers stopped the study early because more women in the HRT group developed breast cancer than women in the placebo group. In the HRT group, 3 extra women per 1,000 developed breast cancer per year compared to the women taking placebo. The researchers followed up all the women in the study for an average of 2.4 years. They found that women who had taken HRT continued to have a higher risk of breast cancer than women who had not taken HRT. The researchers will present more results from this study in the future.

In May 2006, a study was published that looked just at long term use of oestrogen only HRT. The researchers found that the longer women took oestrogen only HRT, the more their breast cancer risk increased. But there was only an increased risk after 15 years use for hormone sensitive breast cancers (oestrogen or progesterone receptor positive) and after 20 years use for all types of breast cancer. The researchers say that the risk of breast cancer after 20 years use is about 40% higher. So, in women taking oestrogen only HRT for 20 years, researchers expect that 49 women would get breast cancer out of every thousand. In women not taking HRT, 35 out of a thousand would be expected to develop breast cancer.

To put all this information in context, a study published in December 2011 estimated that just over 3 out of 100 breast cancers in women in the UK in 2010 were linked to HRT use.This relates to around 1,530 cases. About 3 out of 4 of these extra breast cancers are linked to the use of combined oestrogen progesterone HRT. And 1 out of 4 are linked to oestrogen only HRT.

Women who have had breast cancer are not usually prescribed HRT because studies have shown that it can increase the risk of the breast cancer coming back. But some breast specialists will prescribe it if a woman with breast cancer is having very severe menopause symptoms that nothing else has helped. A clinical trial looking into the risk of HRT after breast cancer stopped recruiting at the beginning of 2004. The women who took part will be followed up by their doctors for the next 5 years at least. They will record the number of women who have a recurrence of breast cancer and compare these figures to women who have not taken HRT. We will not have the results of this trial for some years yet.