Breast cancer patients who take the generic drug tamoxifen for five years are less likely to see their cancer return than those who take it for only two years, according to a large long-term study by British scientists.

The findings showed that for every hundred women with breast cancer who completed a 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.

“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,” said Allan Hackshaw of the Cancer Research UK and University College London Cancer Trials Center, who worked on the study.

“Worryingly our results suggest that by doing this, they could increase their risk of cancer coming back.”

Of the nearly 3,500 patients studied in the trial over 10 years, the cancer came back in around 40 percent of those who took tamoxifen for five years, compared to 46 percent among those who took it for two years.

Breast cancer kills around 500,000 people globally every year and is diagnosed in close to 1.3 million people each year. About 75 percent of these cancers are estrogen-receptor positive, meaning they are driven by hormones.

Tamoxifen was the first drug to block the effects of estrogen, and a new generation of drugs known as aromatase inhibitors has now been developed to act in a similar way.

Experts in the United States estimate that five years of tamoxifen would cost around $8,500, compared with $50,000 to more than $200,000 for chemotherapy to treat breast cancer.

The drug has side-effects. It raises the risk of blood clots, of uterine cancer and of cataracts, according to the U.S. National Cancer Institute; but most experts say its benefits in reducing breast cancer risk outweigh these rarer problems.

This latest British study, which was published in the Journal of Clinical Oncology on Monday, also found that an added benefit of taking tamoxifen for five years was that it reduced the risk of developing or dying from heart disease.

This effect was strongest among women aged between 50 and 59 at diagnosis, with 35 percent fewer women developing a heart condition and nearly 60 percent fewer deaths as a result, they said.

Scientists know that taking tamoxifen for five years gives the best chance of survival from breast cancer. But this trial is the first large study to compare long-term benefit of five years of tamoxifen versus two, over a 10-year follow-up period.

A study in the 1990s by British researchers found that giving tamoxifen to all breast cancer patients who need it, whatever their age, could save an extra 20,000 lives each year worldwide.

SOURCE: Journal of Clinical Oncology, online March 21, 2011.