Older women with early breast cancer should be put on a different drug from the current gold standard, say experts.
Tamoxifen can have serious side effects
Five-year data in the Lancet show anastrozole is better at preventing the spread and return of cancer than tamoxifen in postmenopausal women.
Such women should be put on the drug immediately and any on tamoxifen switched, the authors recommend.
But at about £1,000 a year compared to £20-30 for tamoxifen, it may be some time before it is free on the NHS.
Earlier trial results of anastrozole (Arimidex) versus tamoxifen suggested the newer drug would be superior in some women.
Tamoxifen is still a good treatment option for many women with breast cancer, however.
First line therapy
Currently, women with breast cancer that is responsive to hormonal treatment and who have gone through the menopause are advised to take tamoxifen for five years.
However, tamoxifen can have side effects, including cancer of the womb lining and blood-clotting disorders, and there is still a chance that the breast cancer can spread or return.
Anastrozole is approved for use in any such woman who is unable to take tamoxifen therapy because she has a high risk of experiencing these side effects.
The ATAC trial findings now suggest that the drug should be used by all postmenopausal women with hormone-responsive breast cancer.
In the trial, more than 9,000 such women with cancer confined to the breast were given either tamoxifen or the aromatase inhibitor anastrozole for five years.
Compared with tamoxifen, anastrozole increased disease-free survival by over 10% and increased the time to cancer recurring by around 20%.
It also reduced the chance of the cancer spreading around the body or occurring in the other breast by 14% and over 40%, respectively.
The women taking anastrozole had fewer side effects, although they did report more bone fractures and joint pain than the women taking tamoxifen.
Both anastrozole and tamoxifen work by interfering with the female hormone oestrogen.
Tamoxifen prevents the growth-promoting action of oestrogen on breast cells by blocking a molecule called the oestrogen receptor.
Anastrozole shuts down the production of oestrogen.
Lead investigator Professor Anthony Howell, from the Christie Hospital NHS Trust in Manchester, said earlier results had suggested that it was reasonable to switch patients currently on tamoxifen to an aromatase inhibitor.
"However, these new data from the ATAC trial suggest that it is not appropriate to wait to start an aromatase inhibitor.
"The higher rates of recurrence, and the increased numbers of adverse events and treatment withdrawals associated with tamoxifen, lend support to the approach of offering the most effective and well tolerated therapy at the earliest opportunity."
Dr Sarah Rawlings of Breakthrough Breast Cancer said:
"This is welcome news for postmenopausal women with early stage breast cancer of this type.
"However we would add a note of caution as currently Arimidex is not fully licenced in the UK as an alternative to tamoxifen, and women should not be discouraged from continuing with tamoxifen as it is the best option currently available."
Sue Green, CancerBACUP senior cancer information nurse said: "Women already taking tamoxifen should be assured that the treatment is very effective, but anyone concerned about this news should discuss treatment options with their doctor."
She also urged the watchdog NICE to assess the data urgently so health professionals can be given clear guidance about anastrozole.
Liz Cheesman of Breast Cancer Care said: "It may be some time until the license for anastrozole as a first line treatment is reviewed in the UK and the treatment becomes available for women.
"The challenge facing health professionals is to look at how using anastrazole as a first line treatment compares to other recent trial results showing the benefit of using aromatase inhibitors following tamoxifen as extended treatment."
The study findings will be presented at the San Antonio Breast Cancer Symposium taking place in Texas, the US, this week.
Arimidex is made by AstraZeneca.