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# Aromatase inhibitors

Breast cancer medicines that block aromatase, the enzyme that makes oestrogen from androgens in fat and other tissues after menopause.

Updated 2026-11-01

## What they are

Oral hormonal therapy for hormone receptor-positive breast cancer in postmenopausal women, used after surgery to lower the risk of recurrence and for advanced disease. Two groups: non-steroidal reversible inhibitors (anastrozole, letrozole) and the steroidal irreversible inactivator exemestane.

## Mechanism of action

After menopause the ovaries make little oestrogen; most of it comes from aromatase (CYP19A1), which converts androstenedione and testosterone into estrone and estradiol in fat, muscle and the tumour itself. Blocking aromatase lowers circulating oestrogen and starves oestrogen-dependent tumour cells. In premenopausal women the ovary keeps producing oestrogen, so these drugs alone do not suppress it.

Target: Aromatase (CYP19A1), a cytochrome P450 enzyme

## Key points

- Used for hormone receptor-positive breast cancer in postmenopausal women; ER-negative tumours rarely respond.
- Bone loss is the classic adverse effect: bone mineral density falls (tamoxifen, by contrast, preserves bone in postmenopausal women).
- Joint pain and stiffness, hot flashes and a rise in cholesterol are common.
- Non-steroidal (anastrozole, letrozole) bind reversibly; exemestane is a steroidal false substrate that inactivates the enzyme permanently.
- They can cause fetal harm.

## Drugs in this class

- [**Anastrozole** A breast cancer tablet for postmenopausal women that blocks the enzyme making oestrogen. It can thin the bones and… Aromatase (CYP19A1)](https://durgical.com/en/pharmacology/anastrozole/)
- [**Letrozole** A tablet that blocks oestrogen production, used for breast cancer after menopause. Like the other aromatase… Aromatase (CYP19A1)](https://durgical.com/en/pharmacology/letrozole/)
- [**Exemestane** A steroidal breast cancer tablet that permanently disables the enzyme making oestrogen after menopause. Bone loss is… Aromatase (CYP19A1)](https://durgical.com/en/pharmacology/exemestane/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why are aromatase inhibitors used on their own only after menopause?

Before menopause the ovaries make most of the oestrogen, and lowering oestrogen raises gonadotropins that drive the ovary further. After menopause oestrogen comes mainly from peripheral aromatase, which these drugs block.

## Which bone effect separates aromatase inhibitors from tamoxifen in postmenopausal women?

Aromatase inhibitors lower bone mineral density and raise fracture risk, while tamoxifen acts as an oestrogen agonist on bone and tends to preserve it.

## How does exemestane differ from anastrozole and letrozole at the enzyme?

Exemestane is a steroid that looks like androstenedione; aromatase processes it into an intermediate that binds irreversibly and inactivates the enzyme. Anastrozole and letrozole are non-steroidal and bind reversibly.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Anastrozole: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a696018.html) (MedlinePlus (U.S. National Library of Medicine), 2018); [DailyMed - EXEMESTANE tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1841ec12-3f4f-4b87-b719-fd0b4d9eaffb) (DailyMed (U.S. National Library of Medicine), 2024).

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