Experts are urging breast cancer patients to complete their full prescription of tamoxifen, following long-term results from a major Cancer Research UK-funded trial which showed the cancer was less likely to come back in women who took the drug for five years, compared to two years.

Of the nearly 3,500 patients took part in the study, the cancer came back in around 40 per cent of the women who took tamoxifen for five years, compared to 46 per cent among those who took it for two years, according to results published in the Journal of Clinical Oncology on Monday.

So for every hundred women who completed the full five year course of tamoxifen, the cancer came back in around six fewer women, compared to those who only took the drug for two years.

Scientists already know that taking tamoxifen for five years gives the best chance of survival from breast cancer. But this randomised trial - led by researchers based at the Cancer Research UK and UCL Cancer Trials Centre - is the first large study to compare the long-term benefit of five years of tamoxifen versus two, over a ten year follow-up period.*

Senior author Dr Allan Hackshaw said: "Our study provides conclusive evidence that taking tamoxifen for five years offers women the best chance of surviving breast cancer. Women diagnosed with early stage breast cancer who are prescribed tamoxifen are recommended to take the drug for five years, but we know that many stop after two or three. Worryingly our results suggest that by doing this, they could increase their risk of cancer coming back."

An additional benefit of taking tamoxifen for five years was that it was found to reduce the risk of developing or dying from heart disease. This effect was strongest among women aged 50-59 at diagnosis, with 35 percent fewer women developing a heart condition and nearly 60 per cent fewer deaths as a result.

Dr Hackshaw added: "Ours is the first large study to look at the long-term benefits of five years of tamoxifen and show it has the extra benefit of substantially reducing a woman's risk of developing heart disease. This effect was greatest among women in their 50s, perhaps because the way plaques tend to build up in the arteries with age may be easier for tamoxifen to reverse in younger women."

Kate Law, director of clinical research at Cancer Research UK, said: "This large Cancer Research UK-funded study is the first to look at the long-term performance of tamoxifen - considered to be one of the most important drugs in the history of breast cancer treatment. Our research is behind many important drugs, including tamoxifen, that have contributed to more than three quarters of women with breast cancer now surviving their disease for ten years or more.

"It's vital that doctors and nurses continue encouraging women to finish their course of tamoxifen and providing the necessary support to ensure any side-effects are effectively managed. We would urge anyone who experiences problems taking their medication to consult their doctor without delay."

Source: Cancer Research UK