Medical Oncologist
Medicines
How does exemestane work, and who is it for?
Exemestane lowers oestrogen by permanently switching off aromatase, the enzyme that makes oestrogen after menopause. That takes away a growth signal from hormone-sensitive breast cancer. This page explains the mechanism step by step, why it is called steroidal, who it suits and does not suit, and how it differs from other hormone treatments.
NG
Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
On this page
- How does exemestane work, and who is it for?
- How exemestane affects hormone-sensitive cancer
- Who exemestane may and may not suit
- The vocabulary, in plain language
- Honest realities about how exemestane works
- How exemestane differs from other hormone treatments
- Why exemestane is taken after food
- Checks usually done before exemestane
- What people assume about how exemestane works
- Common questions about how exemestane works
The short answer
How does exemestane work, and who is it for?
Many breast cancers are hormone-receptor-positive, which means their cells carry receptors that respond to oestrogen and use it as a signal to grow. Lowering oestrogen takes that signal away. Before menopause, most oestrogen comes from the ovaries. After menopause, the ovaries stop, but the body still turns other hormones, called androgens, into oestrogen in fat, muscle, skin and breast tissue. This conversion depends on an enzyme called aromatase. Exemestane blocks aromatase. What makes it different from letrozole and anastrozole is the way it does this. Exemestane has a shape similar to the natural hormone that aromatase normally works on. The enzyme mistakes it for that hormone, takes hold of it and is permanently switched off. The body has to make new enzyme, which exemestane then blocks again. For this reason it is sometimes called an aromatase inactivator. The result is the same as with the other two tablets: oestrogen levels fall to very low levels within days. Exemestane therefore suits women whose ovaries are no longer active, either after natural menopause, after surgery to remove the ovaries, or while having injections that switch the ovaries off. It will not help cancers that are hormone-receptor-negative, and it is not enough on its own for women whose ovaries still work, because the ovaries can simply make more oestrogen. Your pathology report and menopause status decide whether it is suitable.
It switches off the enzyme permanently
Exemestane binds to aromatase for good, rather than blocking it temporarily.
It suits women past menopause
Or younger women whose ovaries have been switched off.
The cancer must be hormone-sensitive
It only helps cancers with oestrogen or progesterone receptors.
This page gives general information only. Your oncologist will explain whether exemestane suits your cancer.Step by step
How exemestane affects hormone-sensitive cancer
Four steps take you from the tablet to the cancer cell.
Androgens are made
After menopause, the adrenal glands and ovaries still make androgens, which are the raw material for oestrogen.
Aromatase is switched off
Exemestane locks onto aromatase in body tissues and stops it working permanently.
This is why it is called an irreversible or steroidal inhibitor.Oestrogen falls
With little aromatase working, the amount of oestrogen in the blood and in breast tissue drops sharply.
Growth signals weaken
Hormone-sensitive cancer cells lose a key signal, which can stop hidden cells growing.
What this can mean
- Lower chance of early cancer returning
- Fewer new cancers in the other breast
- Control of advanced cancer for a time
Who it suits
Who exemestane may and may not suit
Situation
General suitability
Woman past menopause, hormone-receptor-positive cancer
A common and suitable choice
Younger woman with ovarian suppression
May be suitable, usually for higher-risk cancer
Younger woman with working ovaries, no suppression
Not suitable, as the ovaries keep making oestrogen
Hormone-receptor-negative cancer
No benefit expected
Advanced cancer after letrozole or anastrozole
Often an option, sometimes with everolimus
Words you will hear
The vocabulary, in plain language
- Aromatase
- The enzyme that turns androgens into oestrogen in body tissues.
- Androgens
- Hormones such as testosterone, made in smaller amounts by women too.
- Oestrogen receptor
- A protein on cancer cells that responds to oestrogen and helps them grow.
- Steroidal aromatase inhibitor
- An aromatase blocker with a hormone-like shape that switches off the enzyme permanently, like exemestane.
- Non-steroidal aromatase inhibitor
- An aromatase blocker that binds reversibly, such as letrozole or anastrozole.
- Menopausal status
- Whether your ovaries are still making hormones, judged by periods, age and sometimes blood tests.
Being straight with you
Honest realities about how exemestane works
Understanding the mechanism helps, but it does not answer every question.
Some cancers find other routes
Over time, some cancers learn to grow without oestrogen. This is called resistance, and it is why advanced cancer treatment may change.
Menopause is not always clear
After chemotherapy, periods may stop and then return. Hormone tests help, but they are not perfect.
Low oestrogen affects the whole body
The same drop in oestrogen that slows cancer also affects bones, joints, the vagina and mood.
What this page cannot tell you
It cannot say how well exemestane will work for your cancer. Your oncologist can explain your situation.
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Compared with others
How exemestane differs from other hormone treatments
Several hormone treatments are used for breast cancer, and each works in its own way.
Letrozole and anastrozole
These also block aromatase, but reversibly. Large trials have found exemestane and anastrozole work similarly well after menopause.
Tamoxifen
Tamoxifen blocks oestrogen receptors on cancer cells instead of lowering oestrogen, so it works before and after menopause.
Fulvestrant
Fulvestrant is an injection that blocks and breaks down the oestrogen receptor, used mainly for advanced cancer.
After another aromatase inhibitor
Because exemestane acts differently, it can sometimes still help advanced cancer that has grown on letrozole or anastrozole, though usually for a shorter time.
A practical point
Why exemestane is taken after food
How a medicine is absorbed affects how much reaches the body. With exemestane, food plays a helpful part.
Absorption improves with a meal
Studies show the body absorbs more exemestane when it is taken after a meal, especially one with some fat.
Consistency matters most
Taking it after the same meal each day keeps levels steady and helps you remember.
Getting started
Checks usually done before exemestane
Because exemestane only works in the right setting, your team confirms a few things first.
Receptor status
Your pathology report must show the cancer is oestrogen or progesterone receptor positive.
Menopause
If there is any doubt, for example after chemotherapy, hormone blood tests may be arranged before starting and repeated during treatment.
Bone health
A bone density scan and vitamin D test give a starting point, so any bone thinning can be spotted and treated early.
Commonly believed
What people assume about how exemestane works
Exemestane kills cancer cells directly, like chemotherapy.
It works by starving hormone-sensitive cells of oestrogen, not by poisoning them.
Steroidal means it builds muscle or raises blood sugar.
The word describes shape. It does not act like cortisone or body-building steroids.
It stops the ovaries working.
It does not. Separate injections or surgery are needed to switch off the ovaries.
If one aromatase inhibitor failed, exemestane cannot help.
In advanced cancer, exemestane can sometimes still work after letrozole or anastrozole.
Questions we are asked
Common questions about how exemestane works
How quickly does exemestane lower oestrogen?
Oestrogen levels fall within the first days of daily tablets and stay low while you keep taking it. The effect on cancer builds over months and years, because the aim is to stop hidden cells growing rather than to shrink a visible tumour quickly.
Why can exemestane not be used alone before menopause?
Working ovaries make large amounts of oestrogen. When aromatase is blocked, the brain signals the ovaries to work harder, which can overcome the tablet. That is why younger women need ovarian suppression alongside it.
What does hormone-receptor-positive mean on my report?
It means the lab found oestrogen receptors, progesterone receptors or both on your cancer cells. The report may give a score or percentage. Cancers with strongly positive results are the ones most likely to respond to hormone treatment.
Does permanent blocking make exemestane stronger?
Not in a way that shows up clearly in trials. In women after menopause, exemestane and anastrozole gave similar results for early breast cancer. The different mechanism matters mainly when one tablet stops working or causes side effects.
Can men take exemestane?
In men, the testes keep making hormones that can be converted to oestrogen, so an aromatase inhibitor alone is not usually enough. Tamoxifen is generally the first choice. Exemestane may be used together with a medicine that lowers testosterone.
Why does exemestane cause joint aches?
The exact reason is not fully understood. Oestrogen helps joints and the tissues around them stay supple, so very low levels can cause stiffness and aching. Exercise, weight management and pain relief help many women.
Does exemestane stop working after a while?
In early breast cancer, it keeps lowering oestrogen for as long as you take it. In advanced cancer, some cancers eventually adapt and grow despite low oestrogen. Scans and symptoms show when that happens, and treatment is then changed.
Does exemestane help progesterone-receptor-positive cancer?
Progesterone receptors are made in response to oestrogen, so cancers positive for them usually respond to lowering oestrogen too. Most such cancers are oestrogen-receptor-positive as well. Your oncologist will interpret your own receptor results.
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Sources
- National Cancer Institute — Hormone therapy for breast cancer
- Cancer Research UK — Exemestane (Aromasin)
- MedlinePlus — Exemestane
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.