Angelina Jolie's decision to have a preventive double mastectomy has made many women think a bit harder about their own risk of breast cancer. Women have an average lifetime risk of developing breast cancer of one in eight, but that figure is considerably higher if you have a family history of the disease and if you are older. Now for the first time, National Institute for Health and Care Excellence (Nice) guidance says that women whose family history puts them at moderate or high risk should be offered tamoxifen, an anti breast-cancer drug, for five years.

This is a big deal, as tamoxifen isn't even licensed for preventive use. Tamoxifen blocks oestrogen receptors on breast cells so that the oestrogen can't reach any existing cancer cells and promote their growth. But it also works (no one quite knows how) on breast cancers that aren't sensitive to oestrogen. It's thought to reduce the risk of breast cancer in this group of women with a family history of the disease by one third. And it's cheap for the NHS at around £25 a year. So do you need it?

The solution

We know that mutations in some genes, such as BRCA1, BRCA2 and TP53, substantially increase the likelihood of breast cancer. Women with Jewish ancestry are five to 10 times more likely to carry BRAC1 or BRAC2 mutations. Between 45% and 90% of women with BRCA genes will develop breast cancer. But regardless of genes, Nice says women with relatives with breast cancer would benefit from being assessed for their own risk. They include women with: first-degree relatives (parents, siblings or offspring) who had breast cancer under the age of 40; two first-degree or one first- and one second-degree who got it at any age; or three first- or second-degree relatives who had breast cancer at any age. If you have relatives with ovarian and breast cancer, you also need to be assessed. Your doctor will gauge your risk using a computer program, and if it comes in at 17% or more and you are over 35 then you may be offered tamoxifen. You may also be offered risk-reducing surgery or regular surveillance with mammograms or MRI scans.

Tamoxifen has side-effects, such as hot flushes and sweats (it acts like the menopause by blocking oestrogen), nausea, weight gain, leg cramps, depression, headaches and blood clots. It can also affect the voice. It is estimated that only 12% of eligible women may decide to take tamoxifen to reduce their risk. It is worth mapping out who in your family has had breast or ovarian cancer, and if you are eligible, see your doctor. Nice guidance also recommends breastfeeding, not smoking and keeping your weight down to reduce your risk.