Of the nearly 3500 patients took part in the study, the cancer came back in around 40% of the women who took tamoxifen for five years, compared to 46% among those who took it for two years, according to results published in the Journal of Clinical Oncology this week.
So for every 100 women who completed the full five-year course of tamoxifen, the cancer came back in around 6 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. However, 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 5 years of tamoxifen versus 2, over a 10-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% fewer women developing a heart condition and nearly 60% 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."
*Nearly 3500 patients aged 50 to 81 diagnosed with operable early breast cancer recruited between 1987 and 1997 participated in the trial. After taking tamoxifen for 2 years, half the women stopped taking the drug and half carried on for a further 3 years. Patients were then followed up for a period of 10 years and the number of recurrences, new tumours, deaths and heart problems recorded.
The cancer came back in 40% of women who took tamoxifen for 5 years compared to 46% among those who took it for 2 years.
An additional benefit of tamoxifen was that it reduced the chance of developing heart disease. This effect was greatest among women aged 50-59 at diagnosis, of which 12% developed heart disease after taking tamoxifen for 5 years, compared to 20% who took the drug for 2 years
Tamoxifen is usually prescribed for women who have oestrogen receptor (ER) positive breast cancer cells. The oestrogen receptor is the part of the breast cancer cell that oestrogen locks on to, to stimulate the cell to multiply. Tamoxifen is able to lock on to the oestrogen receptor and stop the oestrogen from causing cells to grow.
In the 1990s, Cancer Research UK scientists found that giving the drug, tamoxifen, to all breast cancer patients who need it, whatever their age, could save an extra 20 000 lives each year worldwide. This overturned accepted wisdom that the drug had no benefit for younger women.
In 1969, the synthetic oestrogen-blocker tamoxifen was first used to treat breast cancer at the Christie Hospital in Manchester. Tamoxifen is now widely used in breast cancer treatment, and Cancer Research UK has been at the forefront of research into the drug's effectiveness.
In 2002, a Cancer Research UK study, IBIS I, showed that tamoxifen could also be used to prevent breast cancer in high-risk post-menopausal women. However, tamoxifen is not without side effects, so Cancer Research UK is now looking at another anti-oestrogen drug, arimidex, in a study called IBIS II.
Source: Cancer Research UK
