Hormonal therapy tamoxifen halves the risk of women dying from the most common cause of breast cancer when taken over 10 years rather than five years, a new study has shown.

The Cancer Research UK study of 7,000 women with oestrogen receptor positive breast cancer found that patients taking the drug for longer than the recommended five years are better protected against recurrence.

Among women who took tamoxifen for more more than a decade, 25% fewer had recurrences of breast cancer and 23% fewer died, compared to women who were treated with the therapy for five years.

Around 75% of breast cancers are oestrogen receptor positive and may benefit from hormone therapy.

Dr Daniel Rea, clinical lead researcher based at the University of Birmingham commented on the study; "These results are important as they establish that giving tamoxifen for longer than the current standard of five years significantly cuts the risk of breast cancer returning.

"Doctors are now likely to recommend continuing tamoxifen for an extra five years and this will result in many fewer breast cancer recurrences and breast cancer deaths worldwide. Tamoxifen is cheap and widely available so this could have an immediate impact."

Tamoxifen, discovered by AstraZeneca, blocks oestogen receptors and reduces the chance of breast cancer returning after surgery.

Rare but serious side effects of tamoxifen include risk of stroke, but the latest study showed no increase in the incidence of stroke observed over 10 years, though endometrial cancer risk was higher in this arm.

Professor Richard Gray, based at the University of Oxford and presenting the study results at this year’s ASCO conference said; "Five years of tamoxifen is already an excellent treatment but there have been concerns that giving it for longer might not produce extra benefits and could even be harmful. The study establishes that the benefits of taking tamoxifen for longer greatly outweigh the risks."

Image: Tamoxifen is sold under the trade names Nolvadex, Istubal, and Valodex, but is widely referred to by its generic name.