What is anastrozole? All you need to know about the new breast cancer drug

Anastrozole has the potential to reduce the risk of breast cancer by almost 50% for many postmenopausal women at risk, here's what you need to know

Three tablets sitting on a table, representing anastrozole, the new breast cancer drug
(Image credit: Getty Images)

Anastrozole is the name of a drug previously used to treat breast cancer in post-menopausal women. It is now being offered to some women after menopause as a preventative measure, with protective effects proven to last years post-treatment. 

It's thought that an estimated 289,000 women in the UK could benefit from taking anastrozole, with recent trials showing it has the potential to reduce the risk of breast cancer in postmenopausal women with a moderate to high risk of developing the disease by almost 50%. 

Although the drug was recommended by the National Institute for Health and Care Excellence (NICE) as a preventative measure some years ago, it remained a treatment option as it didn't have the license to be used in this way. In November 2023, the Medicines and Healthcare products Regulatory Agency (MHRA) licensed the drug for prevention as scientists in association with Queen Mary University found that the preventative effects of anastrozole could last for years after women have stopped taking the tablet. It's a huge breakthrough in medicine, with the potential for thousands of lives to be changed every year. 

As NHS chief executive Amanda Pritchard said: "It's fantastic that this vital risk-reducing option could now help thousands of women and their families avoid the distress of a breast cancer diagnosis. Allowing more women to live healthier lives, free of breast cancer is truly remarkable, and we hope that licensing anastrozole for a new use today represents the first step to ensuring this risk-reducing option can be accessed by all who could benefit from it."

But what is it and who is eligible for the preventative hormone therapy? Here, we explain all you need to know about the new breast cancer drug, with the help of a breast cancer specialist and surgeon. 

What is anastrozole? 

Anastrozole is an oral 1mg tablet, a form of hormonal therapy, taken once a day for five years. It was previously only approved for use in the UK as a treatment for breast cancer in postmenopausal women but now it has been "repurposed" as the first drug to go through NHS England's Medicines Repurposing Programme.

It is part of a wider group of drugs called aromatase inhibitors, which prevent the release of the enzyme aromatase. After menopause, aromatase is responsible for creating oestrogen, a hormone that can fuel the growth of many types of breast cancer.  

Breast cancer is the most common cancer in the world, with 2.3 million people diagnosed with the disease every year around the world. In the UK alone, there are almost 56,000 new cases of breast cancer diagnosed annually. If even just one in four eligible patients come forward to take anastrozole, the NHS suggests that 2,000 cases of breast cancer may be prevented each year in the UK. It could also save the health service £15 million in treatment costs.

What does anastrozole do to the body? 

Anastrozole works to both treat and prevent breast cancer by targeting levels of oestrogen, the hormone that can contribute to the growth of many different types of breast cancer. 

Before menopause, oestrogen is made in the ovaries. After menopause, it's made by the aromatase enzyme, which is found in the body's fatty tissue - such as the breasts - muscles, and skin. As anastrozole is an aromatase inhibitor, it prevents aromatase from working as it otherwise would to create oestrogen. 

Without aromatase, there is very little oestrogen in the body and without oestrogen, there is no fuel for the cancer cells to feed from, explains Debashis Ghosh, a consultant oncoplastic breast surgeon, based at The Royal Free London. "The whole point of chemoprevention in high-risk individuals, like patients with BRCA1, is to prevent the cancer cells from feeding on oestrogen. Oestrogen feeds cancer cells, allowing them to grow, so we want to eliminate that," he says.

This means anastrozole is highly effective at preventing breast cancer from coming back, or when used as a preventative measure, from developing in the first place.

Debashis Ghosh
Debashis Ghosh

Debashis Ghosh is a breast cancer specialist as a consultant oncoplastic breast surgeon based at The Royal Free London. He is actively involved in breast surgery research including the pioneering use of a portable gamma camera for sentinel node biopsies and in trials based on the use of intraoperative radiotherapy, radioactive iodine and seed localisation. 

Who is eligible for the breast cancer drug?

Post-menopausal women in the UK who are at moderate to high risk of developing breast cancer will be offered the chance to take the drug, the MHRA says. However, the agency has yet to define who sits at this risk level exactly.

Among those at high risk of developing breast cancer who could benefit from anastrozole are women with a defective BRCA1 gene, explains Ghosh. "We all produce cancer cells but we have genes like the BRCA1 that kill these cells. Those who have a defective BRCA1 gene are unable to eliminate these cancer cells and they clump together to form a tumour that grows." It is possible for this gene mutation to be passed on from parent to child and in family lines, so those with a history of breast or ovarian cancer in the family are more likely to be at risk. 

Also considered to be at a moderate to high risk of breast cancer after menopause are those with certain kinds of benign breast disease or those with a high density of breast tissue, according to Cancer Research UK

Anastrozole isn't the only drug available for breast cancer prevention though. Exemestane and letrozole are other options, along with tamoxifen, which has been used in the US from as early as the late nineties as a preventative medicine and is also available with the NHS in the UK. Tamoxifen can be taken regardless of age but it has some serious side effects, including severe menopause symptoms, an increased risk of blood clots, and an increased risk of uterine cancers such as endometrial cancer and uterine sarcoma.

Professor Peter Johnson, the national clinical director for cancer at NHS England told BBC Radio 4's Today programme that this is what makes anastrozole a "very attractive" idea for those with a risk of breast cancer. "People have been particularly concerned about blood clots and also in some cases, the development of endometrial cancer [with tamoxifen]. Anastrozole doesn't seem to do that so it's a more attractive idea," he said.

What are the disadvantages of anastrozole? 

Much like any other drug - from the Covid-19 vaccines to Ozempic for weight loss - there are still side effects to anastrozole. The drug pushes oestrogen levels into a dramatic decline, leading to potentially more severe menopause symptoms. Other side effects include: 

  • Hot flushes
  • Fatigue 
  • Painful or stiff joints 
  • Arthritis 
  • Skin rashes
  • Nausea
  • Headaches
  • Osteoporosis
  • Depression

These symptoms are expected to get worse the longer the drug is taken, which is why the recommended treatment time is currently five years.

However, Ghosh says that once the treatment is over, it's likely that the symptoms will cease. "Menopause symptoms are produced whenever the body is starved of oestrogen," he explains. "A sudden deprivation of oestrogen will always produce menopause symptoms but with time the body adapts and returns to baseline."

Anyone worried about their risk of breast cancer, the lesser-known breast cancer symptoms, and/or possible preventative options should contact their GP, who can explain the options and make a referral to a specialist for a full risk assessment. 

Grace Walsh
Health Channel Editor

Grace Walsh is woman&home's Health Channel Editor, working across the areas of fitness, nutrition, sleep, mental health, relationships, and sex. She is also a qualified fitness instructor. In 2024, she will be taking on her second marathon in Rome, cycling from Manchester to London (350km) for charity, and qualifying as a certified personal trainer and nutrition coach. 

A digital journalist with over six years experience as a writer and editor for UK publications, Grace has covered (almost) everything in the world of health and wellbeing with bylines in Cosmopolitan, Red, The i Paper, GoodtoKnow, and more.