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Older women can halve their risk of breast cancer by taking a new hormone therapy pill, research shows.
The female hormone estrogen can help tumours grow so taking a drug to block it reduces the risk from deadly cancers.
The most popular version is tamoxifen for which 700,000 prescriptions are written on the NHS each year but a new version has been found to better halt tumours.
The drugs are offered to women who have either had breast cancer or are at a high risk of it because of their family history.
A study of almost 4,000 women around the world published in the Lancet found anastrozole improved long term protection for those who take it for five years and then stop.
Up to seven years later breast cancer incidence was 49% among the half randomly assigned to it compared to those given a placebo.
This is significantly better than the 28% decrease recorded for women given tamoxifen.
British scientists say the new drug, prescribed under the brand name Arimidex, should become the “drug-of-choice” for post-menopausal women at increased risk of developing the disease.
Lead author Prof Jack Cuzick, of Queen Mary University of London, said: “This is an exciting finding which makes a strong case for anastrozole being the drug of choice for post-menopausal women at high risk of developing breast cancer.
“Tamoxifen could be offered to the relatively few women who experience serious side-effects from anastrozole.”
Tamoxifen has revolutionised the prevention and treatment of breast cancer.
Researchers also found anastrozole has fewer side-effects, offering an alternative for patients fearing potential hot flushes, irregular periods, tiredness, weight gain or hair thinning.
The first phase of the study published in 2013, called IBIS-1, reported breast cancer occurrence amongst women taking anastrozole fell by 53%.
However the drug is not currently offered to all the women who could benefit because some physicians are unsure of the evidence of its for long term impact.
Co author Dr Ivana Sestak, reader in medical statistics at Queen Mary, said: “The findings mean that for every 29 women taking anastrozole for five years, one case of breast cancer will be prevented during a 12 year period.
“Around 49 women would need to take tamoxifen for five years to prevent one breast cancer case during the same period.”
The findings are presented at the San Antonio Breast Cancer Symposium in Texas.
Professor Cuzick, who is co-chairman of International Breast Cancer Intervention Studies (IBIS), added: “Previous research has confirmed anastrozole is very effective while women are still taking the drug but this is the only trial looking at whether it offers long term protection for women at high risk of developing breast cancer.
“We have concluded that it is highly effective in reducing breast cancer occurrence for at least 12 years.”
Breast cancer is the most common cancer in women and affects over 55,000 UK women each year, killing more than 11,000.
Prof Charles Swanton, Cancer Research UK’s chief clinician, said: “Both tamoxifen and anastrozole can be given to women who are at higher risk of breast cancer.
“Up until now we only knew tamoxifen has long lasting benefits, so it’s reassuring that this study looking specifically at anastrozole, which has fewer long term side-effects, gives better protection to women years after they stopped taking the drug.
“Doctors may still decide that tamoxifen is more appropriate for some women, but it’s great there are options.