The drug tamoxifen could reduce the risk of contracting breast cancer among women who are at high risk of the disease, researchers believe.
Researchers from the Cancer Research UK Centre for Epidemiology, Mathematics and Statistics found that the cancer treatment also has a protective effect for several years after patients stop taking the treatment and believe that the finding is a "major step" towards preventing breast cancer from occurring.
Volunteer women, all of whom had an increased risk of breast cancer, were given 20mg of tamoxifen every day for five years and were found to be substantially less likely to contract the disease than those given a placebo pill.
Just 142 breast cancers were diagnosed among the women after a follow-up of around eight years, compared to 195 in women given the placebo.
Lead researcher Professor Jack Cuzick said that the results confirmed that the drug continued to prevent oestrogen receptor positive breast cancer in women with an increased risk of the disease for at least five years after treatment had stopped.
"Additionally we found that almost all of the excess side-effects reported on tamoxifen do not continue after treatment stops," he added.
The study, which involved 7,154 pre and post-menopausal women, is published in the Journal of the National Cancer Institute.
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