Endocrine (hormone) therapy

Many breast cancers respond to hormones present in the body, such as oestrogen and progesterone. 

If a cancer is oestrogen receptor positive then it grows faster when oestrogen is present.  If a cancer is progesterone receptor positive, then it grows faster when progesterone is present.

Endocrine therapy (sometimes known as hormone therapy) works by blocking the effects of these hormones so that they are unable to stimulate the cancer cells to grow.

Your pathology report will tell you whether your breast cancer is responsive to oestrogen or progesterone or both. If your breast cancer is hormone receptor positive you may benefit from endocrine therapy.

Endocrine therapy is used to reduce the risk of hormone positive breast cancer from coming back following surgery, radiotherapy and/or chemotherapy. It can be used in both pre- and post-menopausal women, but your menopausal status and whether your cancer was node-positive or node-negative will influence your doctor’s decision about which treatment option is right for you. Endocrine therapy is often taken for a long time. You may need to switch from one option to another as time goes by; your doctor will advise you on this.

There are three general categories of endocrine therapy drugs, which are all taken as tablets.

Aromatase inhibitors

These drugs work by blocking the production of oestrogen in the body and are generally only suitable for post-menopausal women. If taken as recommended, aromatase inhibitors have been shown to significantly reduce the risk of breast cancer returning to the same breast or occurring in the opposite breast.

Drugs in this category include anastrozole, letrozole and exemestane. All drugs are taken orally over a long period of time (usually for at least five years).

Side effects of these medicines can include hot flushes, joint and muscle aches, mild nausea, vaginal dryness and reduced bone density, sometimes causing osteoporosis. The potential risk of osteoporosis means that bone density monitoring is recommended while taking aromatase inhibitors.  Most cancer centres in New Zealand now offer regular bone density scans for patients on aromatase inhibitors, but if yours doesn't ask for regular DEXA scans. 

You can read more about dealing with side effects in the section below.

SERMs (Selective Estrogen Receptor Modulators)

This group of drugs prevents oestrogen from taking hold in cancer cells and so reduces the ability of these cells to grow.

The most commonly prescribed SERM in New Zealand is tamoxifen, a drug that can be used in both pre- and post-menopausal women. Again, this drug will need to be taken orally for several years. Recent data has shown benefit from switching post-menopausal women to an aromatase inhibitor following treatment with tamoxifen.

Considerable research effort has been put into finding the optimal duration of endocrine therapies for preventing recurrence and improving survival. Results vary depending on the therapies used and the types of patients enrolled in the studies, but in general, continuing therapy for longer than five years gives better results. For example, patients who took tamoxifen for 10 years had significantly better disease-free survival than those who stopped after five years (ATLAS and aTTom trials), as did those who took tamoxifen for five years and then an aromatase inhibitor for five more years (MA17 trial), and those who took an aromatase inhibitor for five years, or tamoxifen for up to three years, and then followed this with another five years of aromatase inhibitor (NSABP-42 trial). Click here to read an 2025 overview of this topic.

The 2018 American Society of Clinical Oncology (ASCO) guidelines list the different combinations of tamoxifen and aromatase inhibitor treatments that may be recommended by your doctor depending on your menopausal status, the node status of your original tumour, and other breast cancer recurrence risk factors.

Side effects of tamoxifen include menopausal type effects such as hot flushes, and vaginal dryness. More seriously, occasionally (in fact in only one per cent of cases over five years) some women can experience blood clots and very rarely tamoxifen has resulted in uterine cancer. Any vaginal bleeding should be immediately reported to your doctor.

You can read more about dealing with side effects in the section below.

SERDs (Selective Estrogen Receptor Downregulators)

As with SERMs, these drugs work by blocking the effects of oestrogen in the breast tissue so they starve the cancer cells of the oestrogen they need to grow.

At present there is only one SERD available in New Zealand – fulvestrant (brand name: Faslodex®). It is used to treat hormone receptor-positive advanced breast cancer in post-menopausal women with disease progression following other anti-oestrogen therapy, such as tamoxifen or an aromatase inhibitor.

Fulvestrant is given as an injection into a muscle (usually buttocks) once a month. Side effects are generally minimal, however, they can include discomfort at the injection site, hot flushes, vomiting, nausea, and diarrhoea.

Fulvestrant may also be used in conjunction with targeted therapies palbociclib (brand name: Ibrance®) or ribociclib (brand name: Kisqali®) to treat advanced hormone receptor-positive breast cancer. You can read more about these treatments in the section on CDK4/6 Inhibitors on our Targeted Drug Therapy page.

Elacestrant (brand name: Orserdu®) is another SERD, not available or registered in New Zealand, that can be used to treat advanced hormone receptor-positive breast cancer in post-menopausal women (or men) who have a mutation in the ESR1 gene and whose cancer has progressed after other hormone therapy.

Camizestrant is a third SERD, currently being assessed overseas in a clinical trial called CAMBRIA-2.

Other endocrine therapy options

In pre-menopausal women most of the oestrogen and progesterone hormones are produced by the ovaries, and to a lesser degree also in the liver and in fat tissue. A different way of lowering the hormone levels in pre-menopausal women is to remove the ovaries with surgery or by chemically blocking the function of the ovaries with drugs known as lutenising hormone releasing hormone (LHRH) blockers, such as goserelin (Zoladex®) or leuproreline (Lucrin®).

Both drugs are given after chemotherapy as a monthly injection into the abdominal wall. They are fully funded. Both options have a similar effect, i.e. they induce a menopause, but the surgical option is irreversible whereas with the injection option, the pre-menopausal hormone levels will return once the injections are ceased. 

These injections may be recommended for pre-menopausal women with hormone receptor positive breast cancer when taking tamoxifen is not advised or could be used as alternative to tamoxifen.

For more information on LHRH medicines see our fertility section on our pages for young women. You can also watch a video on breast cancer and fertility here.

Dealing with side effects of endocrine therapy

Endocrine therapy for hormone receptor-positive breast cancer prevents recurrence by reducing oestrogen levels and keeping them low so that the cancer is starved of the oestrogen it needs to grow. Not surprisingly, reduced oestrogen levels can lead to menopause-like side effects.

Because endocrine therapy needs to be continued for several years to be fully effective, managing these side effects is very important.

Below we list common side effects associated with tamoxifen and aromatase inhibitors, and some strategies that have been suggested for managing them. Be aware that there is very little high quality scientific evidence for the effectiveness of many of these ‘self-help’ techniques. This doesn’t necessarily mean they won’t work for you. You can read more about this here and here. We have also listed medications that may help; you will need a doctor’s prescription for these.

It’s important to experiment and see what works for you. If none of the self-management techniques work for you, there are medications that could help. Do not hesitate to go back to your doctor and ask for better solutions if you need to.

Hot flushes and night sweats:
-    Wearing clothes made of cotton or linen; avoiding synthetic fabrics
-    Using fans to cool the room and handheld fans to cool your face
-    Wearing light layers of loose clothing that you can take off easily if you need to
-    Sipping cool water regularly
-    Using cooling facial mist sprays
-    Layers of bedclothes that you can remove easily when you wake up hot
-    Cooling bed and/or pillow pads for night sweats
-    Seeing if your hot flushes are triggered by particular foods (e.g. caffeine, alcohol, nicotine, spicy foods) and avoiding them if that is the case
-    Losing weight
-    Physical activity such as yoga
-    Cognitive Behavioural Therapy
-    Mindfulness Based Stress Reduction. Read more here.
-    Medications: SSRIs (selective serotonin reuptake inhibitors) or SNRIs (selective noradrenaline reuptake inhibitors), such as venlafaxine or citalopram; gabapentin; clonidine; oxybutynin.

A new drug called elinzanetant, taken as a daily pill, is also showing promise as a non-hormonal way to reduce the frequency and severity of hot flushes. Read more here. Fezolinetant is a related medicine with similar effects. Neither is registered for use in New Zealand but fezolinetant may be able to be imported and obtained from some pharmacies. Read more here. Fezolinetant was approved (but not yet funded) for the treatment of hot flushes in Australia in April 2024.

Vaginal dryness, itchiness and irritation:
-    Non-hormonal vaginal moisturisers, gels and lubricants (e.g. Replens, YesVM)
-    In some cases, a short course of oestrogen-containing cream may be advised by your doctor. 

-    This article on sexual health after breast cancer may also be of interest.

Joint and muscle pain (aromatase inhibitors only):
-    Acupuncture – some studies support this. Read more here.
-    Physical activity such as yoga, walking, aerobic exercise, resistance training, aquatic exercise or Tai Chi
-    Massage therapy
-    Relaxation techniques
-    Medications (consult your doctor about these): paracetamol; ibuprofen or other non-steroidal anti-inflammatory drugs; opioids if more severe pain; pain modifiers such as tricyclic antidepressants or anticonvulsants for severe resistant pain
-    Consultation with a pain specialist or rheumatologist.

Loss of bone density (aromatase inhibitors only):
-    Regular DEXA scans to monitor bone density
-    Calcium-rich diet
-    Vitamin D (1000-2000 IU per day)
-    Regular weight-bearing and resistance exercise
-    Medications: bisphosphonates; denosumab.

Fatigue:
-    Aerobic exercise 
-    Yoga.

Find out more