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# GnRH analogues (agonists and antagonists)

Injected peptides that switch off the pituitary signal to the testes or ovaries, lowering testosterone or oestrogen: medical castration.

Updated 2026-11-01

## What they are

Synthetic versions of gonadotropin-releasing hormone. Long-acting agonists (leuprorelin, goserelin, triptorelin) and the antagonist degarelix are used for advanced prostate cancer; agonists are also used for hormone-sensitive breast cancer, endometriosis, uterine fibroids and central precocious puberty.

## Mechanism of action

Natural GnRH is released in pulses, and pulses keep the pituitary making LH and FSH. Agonists stimulate the GnRH receptor continuously: first LH, FSH and sex steroids rise (the flare), then the receptors desensitise and LH, FSH, testosterone and oestrogen fall to castration levels. Antagonists block the receptor directly, so hormone levels fall quickly without an initial flare.

Target: GnRH receptor of the anterior pituitary (GNRHR)

## Key points

- Pulsatile GnRH stimulates the pituitary; continuous stimulation by a long-acting agonist shuts it down.
- Agonists cause an initial flare: in prostate cancer bone pain, urinary obstruction or spinal cord compression can worsen in the first weeks, so an antiandrogen is often given at the start.
- Antagonists such as degarelix lower testosterone without a flare.
- Effects of low sex steroids: hot flashes, loss of libido, erectile dysfunction, bone loss, and in men higher risk of diabetes and cardiovascular events.
- Androgen deprivation can prolong the QT interval.

## Drugs in this class

- [**Leuprorelin** An injected hormone that, given continuously, switches off testosterone and oestrogen. It treats prostate cancer… Gonadotropin-releasing hormone receptor (GnRH-R)](https://durgical.com/en/pharmacology/leuprorelin/)
- [**Goserelin** A small implant placed under the skin of the abdomen that releases a GnRH agonist for weeks. It treats prostate and… Gonadotropin-releasing hormone receptor (GnRH-R)](https://durgical.com/en/pharmacology/goserelin/)
- [**Triptorelin** A long-acting GnRH agonist injection for advanced prostate cancer and early puberty in children. Hormones rise for a… Gonadotropin-releasing hormone receptor (GnRH-R)](https://durgical.com/en/pharmacology/triptorelin/)
- [**Degarelix** A monthly injection for advanced prostate cancer that blocks the pituitary GnRH receptor. Testosterone falls quickly… Gonadotropin-releasing hormone receptor (GnRH-R)](https://durgical.com/en/pharmacology/degarelix/)

## Others in the class

- histrelina
- nafarelina
- buserelina

## Test yourself

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

## Why does a drug that stimulates the GnRH receptor end up lowering testosterone?

Continuous stimulation, unlike the natural pulses, desensitises and down-regulates pituitary GnRH receptors; after an initial rise, LH and FSH fall and the testes stop making testosterone.

## What is the tumour flare with GnRH agonists, and how is it prevented?

A temporary rise in testosterone in the first weeks that can worsen bone pain or urinary obstruction in prostate cancer; an antiandrogen is often given at the start, or an antagonist such as degarelix is used.

## Which metabolic risks are linked to GnRH agonists in men?

Hyperglycaemia and new or worsening diabetes, and a higher risk of myocardial infarction, sudden cardiac death and stroke.

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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: [Leuprolide Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a685040.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [Degarelix Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a609022.html) (MedlinePlus (U.S. National Library of Medicine), 2023); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 46 (McGraw Hill, 2023).

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