A pill can ward off breast cancer. Why aren’t more women taking it?

We have long been familiar with the idea of healthy people taking a statin to reduce their risk of cardiovascular disease. What is less well known is a pill that does the same for breast cancer.

The drug, tamoxifen, has a publicity problem. Less than 10 per cent of those women for whom the drug is recommended are actually taking it. This statistic, published earlier this year, led to headlines such as ‘Breast cancer pill denied to 500,000 women.’

The back story of tamoxifen is an interesting one. It’s an example of how older drugs are being used in new ways to prevent cancer.

It has been part of breast cancer treatment for decades. The drug is prescribed for breast cancers classed as oestrogen-receptor (or ER) positive, the most common type of breast cancer. It’s an example of a selective oestrogen receptor modulator, or SERM, and works by binding to oestrogen receptors and interfering with oestrogen’s ability to fuel these cancers.

Researchers found tamoxifen could reduce the risk of new cancers developing in breasts of women previously diagnosed with cancer — both in the affected and unaffected breast. This revealed the potential ability of tamoxifen not only to treat but also to prevent breast cancer.

The International Breast Cancer Intervention Study 1 (IBIS-1) investigated tamoxifen’s breast cancer prevention possibilities in women with a higher risk of breast cancer (based on their family history), but without any personal history of the disease. In this trial, half of the women were given 20mg tamoxifen as a daily dose, and the other half were given a placebo. Women in both groups were instructed to continue their allocated medication for five years.

The findings were remarkable. In these high risk women, tamoxifen reduced the risk of breast cancer by about 30 per cent and, what’s more, the protective effects were found to last at least 10 years.

In 2013 a large study involving more than 80,000 women combined the results of several trials evaluating tamoxifen, including the IBIS-1 trial, and the findings were similar. The women taking tamoxifen had a 33 per cent reduced risk of breast cancer compared to those who didn’t. And again the benefits persisted, with a reduced risk lasting at least five years after treatment ended.

Since 2013 clinical guidelines have recommended that women at either a moderate or high risk of breast cancer should be offered tamoxifen for five years. These women have a higher-than-average risk of breast cancer based on either family history (a close relative such as a mother, sister or grandmother having had breast or ovarian cancer, especially if at a young age), or because of a known BRCA1/BRCA2 genetic mutation.

Despite these recommendations, uptake of breast cancer prevention medication is low.

There’s no definitive reason why this might be the case, but research suggests factors related to both doctors and patients might play a role. The decision to start tamoxifen is arguably more complex and emotive than, say, to start on a statin.

For example, it needs to be considered alongside other breast cancer prevention interventions, like risk-reducing mastectomy. Reproductive considerations also come into play, as tamoxifen shouldn’t be taken within two months of trying to conceive.

The US preventive services task force say doctors should engage women at an increased risk of breast cancer and participate in ‘shared, informed decision making’ about the risks and benefits of taking tamoxifen.

And that’s important. As with all medications, tamoxifen carries some risks. It’s associated with an increased risk of womb (endometrial) cancer, and blood clots in veins. (In the IBIS-1 study, which followed more than 7,000 women, 15 were diagnosed with endometrial cancer in the tamoxifen group as opposed to four in the placebo group; blood clots increased from 25 in the control group to 46 in the tamoxifen group.) Tamoxifen shouldn’t be prescribed to women with a past history of either of these conditions or to those who might be at higher risk of them. It also comes with less serious but unpleasant side effects, like hot flushes, and nausea and vomiting.

It’s important to note that another breast cancer prevention medication has come on the scene more recently. It’s called anastrozole and, while similarly effective at reducing risk, can only be taken by women who have had their menopause. It has fewer side effects, though, and doesn’t have an increased risk of womb cancer or clots.

If you think you might be at a higher risk of breast cancer based on your family history, it’s important to speak to your GP. Doctors can help you weigh up whether tamoxifen is the right option for you, how you personally might benefit, and what your individual risk might be.

It’s important to remember tamoxifen isn’t recommended for women without a higher-than-average risk of breast cancer.

But there are still things all women can do to help reduce their risk, such as keeping a healthy weight, cutting down on alcohol, and being physically active. And, like statins, these things can help your heart to stay healthy too.

Dr Jasmine Just is Cancer Research UK’s health information officer.