Only one in seven women who are at high risk of breast cancer are taking a preventative drug, a new study suggests.
Researchers found that six in seven women with a family history of breast cancer opt out of taking tamoxifen as a preventative medication.
In 2013, the National Institute for Health and Care Excellence (Nice) recommended that certain women at high risk of breast cancer should be given a course of the drug to help slash their chances of developing breast cancer.
It cited a previous trial which found that tamoxifen, taken for five years, reduced the risk of developing invasive breast cancer by about 50% in post-menopausal women who were at increased risk of getting the disease.
The move was hailed as a "historic step for prevention" and it was estimated that almost half a million women would be eligible for treatment.
The latest study, published in the journal Breast Cancer Research and Treatment, found that just 14.7% of eligible women took the drug.
Researchers from the University of Leeds, Northwestern University, University College London and Queen Mary University of London found that women with children were more likely to take the medication.
The study, funded by Cancer Research UK, involved 258 healthy women across England who had been identified as having an increased risk of the disease between September 2015 and December 2016.
The women, who had an average age of 45, were asked whether they had agreed to take the drug to help prevent breast cancer developing.
Just 38 had initiated tamoxifen treatment.
Women who had children were more likely to initiate chemoprevention than those without children - 17.6% compared with 3.8% respectively.
A further 16 women were asked about their decision in more detail and researchers found that having children was a big factor in decision-making - some women opted to take the drug to be "around for" their families while others said that they had too many commitments to take a drug and risk possible side effects.
Meanwhile, wider family beliefs about medication also seemed to have an effect, the authors noted - "distrust of medications within the family resulted in more negative attitudes and beliefs towards tamoxifen uptake".
Others reported a lack of control over cancer risk, and some told researchers they felt they were "predestined" to develop cancer due to multiple family members developing and dying from cancer.
Researchers found no socio-demographic differences in uptake.
Dr Samuel Smith, study author from the University of Leeds, said: "While it's reassuring a woman's background doesn't seem to be a barrier to taking tamoxifen, only one in seven of those at increased risk of breast cancer are taking up the option.
"Therefore it's important doctors can discuss women's concerns and provide information to help them while they are considering their options.
"Further research is needed to understand if all women eligible to take tamoxifen for prevention are getting the help and support they need."
Dr Richard Roope, Cancer Research UK's senior clinical adviser, added: "When an established drug like tamoxifen is found to work not only as a treatment for breast cancer, but is also shown to reduce the risk of the disease, it seems we're making real progress.
"It's valuable to understand why women might reject tamoxifen, and this research highlights there are a range of complex reasons behind the decision.
"It's vital more work is done to understand these barriers, improve treatments and ensure doctors are getting the support they need to help women decide whether preventative medication is right for them.
"Whatever a woman's risk of developing breast cancer, keeping a healthy weight and cutting back on alcohol are also ways of reducing it."
In 2017, Nice updated its breast cancer prevention recommendations to say that certain women with a family history of breast cancer should be offered anastrozole, not tamoxifen, to prevent breast cancer, saying that it would reduce the number of breast cancer cases further still.