The study looked at almost 4,000 postmenopausal women at high risk of breast cancer with half being given 1mg of anastrozole daily and half given a placebo. In the five years of follow up 40 women in the anastrozole group developed breast cancer compared to 85 women placebo group.
Professor Jack Cuzick, lead researcher and head of Queen Mary University of London's Centre for Cancer Prevention, said: "This research is an exciting development in breast cancer prevention. We now know anastrozole should be the drug of choice when it comes to reducing the risk of breast cancer in postmenopausal women with a family history or other risk factors for the disease. This class of drugs is more effective than previous drugs such as tamoxifen and crucially, it has fewer side effects.
"Unpleasant side effects such as acute aches and pains have often been associated with oestrogen depriving drugs. However, in this study, the reported side effects were only slightly higher than in the placebo arm. This means most symptoms were not drug related, and the concern about side effects for this type of drug may have been overstated in the past.
"Our priority now is ensuring that as many women as possible can benefit from these new findings. Prevention is an important tool in the fight against breast cancer and we strongly urge the National Institute for Health and Care Excellence (NICE) to consider adding anastrozole to their recommended drugs for women who are predisposed to developing breast cancer. By including this drug in their clinical guidelines, more women will benefit from this important advancement in preventive medicine."
Many breast cancers are fuelled by the hormone oestrogen. Anastrozole works by preventing the body from making oestrogen and has for many years been used to treat postmenopausal women with oestrogen receptor positive breast cancer.
Women were judged to be at high risk of breast cancer if they fulfilled one of the following criteria: having two or more blood relatives with breast cancer, having a mother or sister who developed breast cancer before the age of 50, and having a mother or sister who had breast cancer in both breasts, or having certain high risk types of benign breast disease.
Kate Law, director of clinical research at Cancer Research UK, said: "This landmark study shows that anastrozole could be valuable in helping to prevent breast cancer in women at higher than average risk of disease. We now need accurate tests that will predict which women will most benefit from anastrozole and those who will have the fewest side-effects."
Dr Harpal Kumar, Cancer Research UK's chief executive, said: "Cancer Research UK has been funding the IBIS II trial for more than 10 years. We're delighted that the results will now offer women at high risk of breast cancer a new option to significantly reduce their chance of developing the disease. By supporting large trials such as this, which take a long time to complete, we can make discoveries that will help us to save many more lives in the future."