# GnRH Analogues and Antagonists in Gynaecology

> NEET-PG Obstetrics and Gynaecology notes on GnRH analogues and antagonists covering agonist flare, add-back therapy and use in fibroids, endometriosis and IVF.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/gnrh-analogues-and-antagonists-in-gynaecology
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Williams Gynecology; Dutta's Textbook of Gynecology
- How to cite: "GnRH Analogues and Antagonists in Gynaecology", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/gnrh-analogues-and-antagonists-in-gynaecology

## Direct answer

GnRH analogues are synthetic peptides that act on pituitary GnRH receptors, where agonists cause a transient flare followed by receptor down-regulation, while antagonists block the receptor and suppress gonadotrophins without flare. In gynaecology they treat endometriosis, shrink fibroids before surgery and control ovarian stimulation in IVF. Hypo-oestrogenic effects such as bone loss and hot flushes are limited by add-back therapy.

## What you must remember

- **Agonist flare:** the first days of agonist treatment produce a surge of LH and FSH that can briefly worsen pain or bleeding, and pretreatment is sometimes used to blunt it.
- **Down-regulation:** continuous agonist exposure desensitises pituitary receptors, producing a menopause-like hypo-oestrogenic state within a few weeks.
- **Antagonists:** oral and injectable antagonists suppress gonadotrophins immediately without flare, and availability varies by country.
- **Add-back therapy:** low-dose oestrogen, with or without progestogen, protects bone density and reduces vasomotor symptoms, and is standard for prolonged use.
- **Bone effect:** prolonged hypo-oestrogenism reduces bone mineral density, so agonist treatment is usually limited to about six months unless add-back is given.
- **IVF use:** agonists serve in long protocols and as a trigger for final oocyte maturation, while antagonists prevent a premature LH surge in flexible protocols.

## Comparing agonists and antagonists

A 34-year-old woman with severe endometriosis is due to start hormonal suppression, and her clinician must choose between an agonist and an antagonist. Both reduce oestrogen, but their onset, route and side-effect profiles differ. The table compares the two classes on the features that most often guide selection.

| Feature | Agonist | Antagonist |
|---|---|---|
| Action | Receptor stimulation, then down-regulation | Direct receptor blockade |
| Initial effect | Flare before suppression | Immediate suppression, no flare |
| Route | Depot injection or nasal | Oral or injection, depending on the agent |
| Gynaecology use | Endometriosis, fibroids, long IVF protocols | Fibroids, endometriosis, IVF antagonist protocols |
| Main limitation | Flare and bone loss | Availability and cost |

Antagonists avoid the flare, which is an advantage when symptoms must not worsen at the start of treatment. Agonists have the longer track record in gynaecology, and add-back therapy is most clearly established with them.

Baseline bone density is considered before prolonged treatment, because the bone effect is cumulative and matters most in young women. In practice the choice also depends on the indication, the expected duration and local availability, and women needing longer suppression are usually offered add-back therapy.

## Where the exam tries to catch you

Examiners test whether you can plan the flare, limit bone loss and use GnRH drugs correctly in IVF.

- **Trap:** Start a GnRH agonist without considering the flare in a woman with heavy bleeding. **Correct:** The initial flare may worsen bleeding or pain, so pretreatment or careful timing is used.
- **Trap:** Continue an agonist indefinitely without add-back. **Correct:** Prolonged suppression causes bone loss, so duration is limited or add-back is added.
- **Trap:** Assume add-back therapy abolishes the benefit of the drug. **Correct:** Add-back is designed to protect bone while preserving symptom control, though books differ on how much benefit is retained in some indications.
- **Trap:** Use an agonist trigger in every IVF cycle. **Correct:** An agonist trigger reduces ovarian hyperstimulation risk in selected antagonist cycles and needs luteal support.

## Frequently asked questions

### What is the GnRH agonist flare effect?

The GnRH agonist flare effect is a transient rise in LH and FSH during the first days of treatment, before pituitary receptors become desensitised. It can briefly worsen pain or bleeding, which is why some protocols use pretreatment to blunt it.

### Why is add-back therapy used with GnRH analogues?

Add-back therapy with low-dose oestrogen, with or without progestogen, is used alongside GnRH analogues to protect bone density and reduce hot flushes. It allows longer treatment than would otherwise be safe in women receiving GnRH analogues.

### How do GnRH antagonists differ from agonists in IVF?

GnRH antagonists in IVF block the receptor directly and prevent a premature LH surge without the initial flare seen with agonists. They are commonly used in flexible protocols, and combined with an agonist trigger they reduce the risk of ovarian hyperstimulation.

### Which gynaecological conditions use GnRH analogues?

GnRH analogues are used in endometriosis, to shrink fibroids before surgery, and in IVF to control ovarian stimulation. Adenomyosis is another recognised indication, and paediatric use for precocious puberty is also established.