A trial comparing anastrozole and tamoxifen for breast cancer (ATAC trial)

Cancer type:

Breast cancer




Phase 3

This trial compared 2 hormone therapies for women with post menopausal early stage breast cancer. This trial was supported by Cancer Research UK.

Doctors usually treat breast cancer with surgery, sometimes followed by hormone therapy. Many breast cancers are hormone receptor positive (or hormone sensitive) cancers. This means that the hormones oestrogen and progesterone encourage these cancers to grow. Drugs that block these hormones can help stop early stage breast cancer coming back after surgery.

When this trial started, the hormone therapy tamoxifen was the standard treatment for women who had had surgery for early breast cancer. It works very well, but doctors wanted to see if they could stop even more cancers coming back after surgery. This trial compared tamoxifen with anastrozole (Arimidex), another type of hormone therapy called an aromatase inhibitor, and a combination of these two drugs.

The aims of this trial were to find out

  • If anastrozole was better at stopping breast cancer coming back than tamoxifen
  • If a combination of anastrozole and tamoxifen was better than tamoxifen alone
  • More about the side effects

Summary of results

The research team found that anastrozole did help stop breast cancer coming back more than tamoxifen, and caused fewer side effects.

This trial recruited 9,366 women who had been through the menopause and had surgery for early stage breast cancer.

  • 3,125 had anastrozole
  • 3,116 had tamoxifen
  • 3,125 had tamoxifen and anastrozole

Tamoxifen and anastrozole are both tablets that you take once a day, every day for 5 years.

The research team discovered quite early on that taking tamoxifen and anastrozole together was no better than taking tamoxifen alone. So this part of the trial was stopped. The women who had been taking both went on to take one or the other.

When the trial started researchers didn’t know for certain that hormone therapies such as tamoxifen and anastrozole worked best for women with hormone receptor positive breast cancer. So not everyone taking part in this trial had hormone receptor positive cancer.

When they first started to look at how well the treatments worked, they discovered that they worked best in the 5,216 women who had hormone receptor positive breast cancer. Of these,

  • 2,618 women were taking anastrozole
  • 2,598 women were taking tamoxifen

The research team looked at how good the treatments were at stopping the cancer coming back. They found that 10 years after starting treatment, the cancer had come back in

  • About 1 in 5 women (19.7%) who had taken anastrozole
  • About 1 in 4 women (24%) who had taken tamoxifen

They also looked at the side effects and found that overall anastrozole caused fewer side effects than tamoxifen. The women taking anastrozole were less likely to have problems with vaginal bleeding and discharge, blood clots in the veins (deep vein thrombosis, or DVT) and hot flushes.

The women taking tamoxifen were less likely to have problems with weakening of the bones (osteoporosis) and fractures while they were having treatment. But there was less of a difference between the 2 groups after they had finished treatment.

The risk of developing a second cancer was low for both groups, but for most cancers it was slightly lower for the women taking anastrozole compared to tamoxifen. In particular, the risk of developing cancer in the other breast was lower for women taking anastrozole.

There doesn’t seem to be a significant difference in how long women live after taking these 2 treatments, but it’s really too early to tell yet. There may be differences between the groups in time.

The research team concluded that anastrozole is better than tamoxifen at stopping early stage breast cancer coming back after surgery. And that it caused fewer serious side effects.

We have based this summary on information from the team who ran the trial. The information they sent us has been reviewed by independent specialists (peer reviewed Open a glossary item) and published in a medical journal. The figures we quote above were provided by the trial team. We have not analysed the data ourselves

Recruitment start:

Recruitment end:

How to join a clinical trial

Please note: In order to join a trial you will need to discuss it with your doctor, unless otherwise specified.

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Chief Investigator

Prof J Cuzick
David Cameron

Supported by

Cancer Research UK
Experimental Cancer Medicine Centre (ECMC)
University of Edinburgh

Other information

This is Cancer Research UK trial number CRUK/95/003.

Questions about cancer? Contact our information nurses

Freephone 0808 800 4040

Last review date

CRUK internal database number:

Oracle 1738

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Harriet wanted to try new treatments

Picture of Harriet

“I was keen to go on a clinical trial. I wanted to try new cancer treatments and hopefully help future generations.”

Last reviewed:

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