GnRH agonists, antagonists and hCG: the peptides that run IVF

Why IVF needs peptides at all

A natural menstrual cycle usually matures one egg. IVF aims for more, because not every egg fertilises and not every embryo develops, and several eggs give a better chance of at least one healthy embryo. IVF stimulates the ovaries with follicle-stimulating hormone so several eggs mature at once, and that creates two problems that peptide drugs solve. First, the brain may release a surge of luteinising hormone (LH) too early and ovulate the eggs before they can be collected. Second, at the right moment the eggs need a signal to finish maturing before retrieval.

Both jobs run through gonadotropin-releasing hormone (GnRH), which makes IVF one of the clearest cases of peptide drugs used exactly as designed. GnRH is a ten-amino-acid peptide from the hypothalamus that tells the pituitary to release LH and FSH. Synthetic versions of GnRH, and drugs that block it, are the tools; hCG, a pregnancy hormone that acts like LH, is the classic trigger.

Stopping early ovulation: agonists and antagonists

There are two ways to stop the premature surge. A GnRH agonist, such as leuprolide or triptorelin, first stimulates the pituitary and then, with continued use, shuts it down; the classic long protocol starts it in the cycle before stimulation. A GnRH antagonist, such as cetrorelix or ganirelix, blocks the pituitary's GnRH receptors directly and works within hours, so it can be started partway through stimulation. A 2016 Cochrane review describes the usual antagonist regimens, including 0.25 mg a day from around day six of stimulation (systematic review) ¹.

Both classes are peptides closely modelled on GnRH itself, with a few amino acids swapped to make them last longer or to block rather than activate the receptor. All four are approved medicines. Cetrotide (cetrorelix) was approved in the US in 2000 for inhibiting premature LH surges in women undergoing ovarian stimulation, and ganirelix is marketed as generics with a label dose of 250 mcg once daily until the day of the trigger (dataset) ². Our entries on cetrorelix and ganirelix give the details.

What the trials show: antagonists vs the long agonist protocol

This is one of the best-studied questions in fertility medicine. The 2016 Cochrane review pooled 73 randomised trials with 12,212 women. Live-birth rates showed no conclusive difference between antagonists and the long agonist protocol (odds ratio 1.02): if 29% of women had a live birth with the agonist, 25% to 33% would with the antagonist. Ovarian hyperstimulation syndrome was less common with antagonists (odds ratio 0.61), cutting an assumed 11% risk to 6% to 9%, and fewer cycles were cancelled for OHSS risk, though more were cancelled for poor response (systematic review) ¹.

A large Danish trial, run at Hvidovre University Hospital in Copenhagen, gave a concrete picture. Among 1,050 women in their first IVF or ICSI cycle, severe OHSS occurred in 5.1% with an antagonist against 8.9% with an agonist, hospital admissions for OHSS were 1.7% against 3.6%, and live-birth rates were similar at 22.8% and 23.8% (randomised trial) ³. That combination, similar success and less OHSS, is why antagonist protocols have become common.

Why many clinics moved to antagonists

The long agonist protocol was the standard for years, and it works, but it has costs that the antagonist approach avoids. Because an agonist first stimulates the pituitary before suppressing it, it causes an initial 'flare', and the suppression takes a long run-in, with a period of low oestrogen and the side effects that come with it. Antagonists act within hours by blocking the receptor directly, so they can be started partway through stimulation, avoiding the flare, the low-oestrogen side effects and the long down-regulation period (systematic review) ¹.

Shorter treatment, fewer injections and less OHSS, with similar live-birth rates, made the antagonist protocol attractive, and it also opened the door to the agonist trigger, which only works when the pituitary has not already been shut down by a long agonist course.

Reading the numbers for yourself

The Cochrane figures are easiest to understand in absolute terms. Switching from a long agonist protocol to an antagonist would leave the chance of a live birth around the same, roughly 29% against 25% to 33% in the review's example, while lowering an 11% risk of any OHSS to 6% to 9% (systematic review) ¹. Switching the trigger from hCG to a GnRH agonist in a fresh transfer would cut a 5% OHSS risk to at most 2%, but could lower a 31% chance of live birth to between 12% and 24% (systematic review) ⁴. Your own numbers depend on age, egg reserve and diagnosis, which is why clinics tailor these decisions. Asking your clinic for its own figures for women like you is reasonable and useful.

What OHSS is, and why it drives these choices

Ovarian hyperstimulation syndrome is the main serious complication of IVF, and avoiding it shapes almost every decision in this article. Stimulated ovaries swell and fluid shifts out of blood vessels into the abdomen, causing bloating and pain in mild cases and, in severe ones, dangerous fluid build-up, blood clots and kidney problems. The Danish trial counted women who needed fluid drained from the abdomen: none in the antagonist group against 2.0% with the agonist (randomised trial) ³.

The trigger matters because the risk peaks after it. The trial evidence below shows that replacing hCG with a GnRH agonist trigger cuts OHSS sharply (systematic review) ⁴, which is consistent with hCG continuing to stimulate the ovaries after the injection while an agonist-induced surge is brief. Much of the choice of protocol and trigger is about managing that risk for each woman.

Triggering maturation: hCG or a GnRH agonist

The traditional trigger is an injection of hCG, given about 36 hours before egg collection. Ovidrel, a recombinant hCG, is approved for final follicular maturation in assisted reproduction, and urinary products such as Pregnyl and Novarel are also used (dataset) ². Our hCG entry covers its other uses.

In women on an antagonist protocol, a dose of GnRH agonist can trigger instead, by provoking a short natural LH surge. A 2014 Cochrane review of 17 trials found the trade-off clearly. In fresh transfers, the agonist trigger cut OHSS sharply (odds ratio 0.15: a 5% risk with hCG fell to between 0% and 2%), but live births were lower (odds ratio 0.47: a 31% chance with hCG became 12% to 24%), and early miscarriage was higher (systematic review) ⁴. The surge is short, and the lower live-birth rate in fresh transfers is thought to reflect weaker hormonal support of the womb lining afterwards.

That is why clinics often use an agonist trigger for women at high OHSS risk and then freeze all the embryos for transfer in a later cycle, or add support to the luteal phase, rather than using it for everyone.

Recombinant or urinary hCG?

hCG for triggering comes in two forms: purified from the urine of pregnant women, or made by recombinant technology. A 2016 Cochrane review of 18 trials with 2,952 women found no conclusive difference in ongoing pregnancy or live birth between recombinant and urinary hCG (odds ratio 1.15) and no evidence of a difference in OHSS, while noting that drug companies funded half the studies (systematic review) ⁵. For most women, the choice between them is practical rather than a matter of outcomes.

High responders: the newest comparison

Women who produce many eggs, such as many with polycystic ovary syndrome, are at the highest OHSS risk. A 2026 network meta-analysis of seven high-quality trials in 632 such women compared the hCG trigger, the GnRH agonist trigger and a 'dual' trigger combining both. Egg numbers, maturity and pregnancy rates did not differ significantly, but the agonist trigger reduced moderate to severe OHSS compared with hCG (risk ratio 0.23) (meta-analysis) ⁶. The dual trigger's OHSS reduction was not statistically clear.

For kisspeptin, the Imperial group also tested a second dose ten hours after the first in high-risk women: more women reached a good egg yield with two doses (71% against 45%) without more OHSS (randomised trial) ⁷. These remain research results from one centre, and larger multicentre trials would be needed before kisspeptin could be considered for approval as a trigger.

An alternative that is not a peptide

Not every protocol uses a peptide to stop early ovulation. Progestin-primed ovarian stimulation, which uses an oral progestogen instead, has been compared with GnRH antagonists in randomised trials. A 2026 meta-analysis of 20 trials with 4,430 women found a modestly higher live-birth rate with the progestogen approach (risk ratio 1.13), though no trial was at low risk of bias and five raised trustworthiness concerns (meta-analysis) ⁸. The evidence is still settling, and the choice depends on the clinic's approach to freezing and transferring embryos.

The 'dual trigger'

Some clinics give a GnRH agonist and a small dose of hCG together, hoping to combine the agonist's natural surge with hCG's support for the womb lining. A 2017 meta-analysis of four trials with 527 women on antagonist protocols found a higher pregnancy rate with the dual trigger than with hCG alone (relative risk 1.55), with no difference in egg numbers or embryo quality, and its authors called for larger trials (meta-analysis) ⁹. It is a common practice with a modest evidence base, a reasonable thing to ask your clinic about.

The same drugs beyond fertility

The GnRH agonists used in IVF are workhorses elsewhere in medicine. Leuprolide, for example, is approved in the US in long-acting forms for advanced prostate cancer, for endometriosis and fibroids before surgery, and for central precocious puberty, where shutting down the pituitary pauses early puberty (dataset) ². Those uses rely on the same pituitary shutdown that stops early ovulation in an IVF cycle, held for months rather than days. Our leuprolide entry and the kisspeptin vs gonadorelin comparison cover the wider family.

Kisspeptin: a more natural trigger in testing

Kisspeptin is the peptide that sits upstream of GnRH, the brain's own switch for the hormone cascade. Researchers at Imperial College tested kisspeptin-54 as a trigger in 60 women at high risk of OHSS, after a standard antagonist protocol, in a phase 2 randomised trial of different doses. Eggs matured in 95% of women; across all doses, live-birth rates per transfer were 45%, and no woman developed moderate, severe or critical OHSS (randomised dose-finding trial) ¹⁰.

A 2017 review called kisspeptin a promising trigger while noting that further studies are needed to establish its place (review) ¹¹. It is not approved as an IVF drug. Our kisspeptin entry covers the research, including the shorter kisspeptin-10 used in other studies.

Peptides sold online for fertility

Kisspeptin-10, gonadorelin, hCG and other hormone peptides are sold online as research chemicals, sometimes marketed for fertility or for men's hormone levels. That is a different thing entirely from the approved drugs above, and our kisspeptin and fertility article explains why the research on kisspeptin does not translate into a home protocol. IVF medicines are dosed to the hour under ultrasound and blood-test monitoring, because the margin between too little and dangerous overstimulation is narrow; an unregulated vial of uncertain content, used without monitoring, removes every safeguard that makes the procedure safe.

hCG also has a long history of misuse for weight loss. The US labels for Pregnyl and Novarel state it has not been shown to be effective for obesity (drug label) ¹².

A note on men

The same hormone system matters for male fertility. hCG is approved in the US for selected cases of hypogonadotropic hypogonadism in men (dataset) ², a condition where the brain does not signal the testes, and fertility specialists use it to restart sperm production in that setting. That use is supervised, with hormone levels and semen tested along the way, and it is prescribed for a diagnosed condition; it is not a reason to buy research hCG to offset steroid use, which is a common online claim with real risks.

Questions to ask your fertility clinic

• Which protocol are you suggesting, agonist or antagonist, and why for me?
• What is my risk of OHSS, and how will the trigger choice reduce it?
• If you use an agonist trigger, will you freeze all embryos or add luteal support?
• How will you monitor my response during stimulation?
• Are any of my medicines experimental, and if so, is that within a trial?

Quick glossary

• GnRH: the hypothalamic peptide that drives LH and FSH release.
• GnRH agonist: leuprolide, triptorelin; stimulates and then suppresses the pituitary.
• GnRH antagonist: cetrorelix, ganirelix; blocks the pituitary's GnRH receptor within hours.
• Trigger: the injection that finishes egg maturation before collection: hCG, a GnRH agonist or, in trials, kisspeptin.
• OHSS: ovarian hyperstimulation syndrome, the main serious complication of IVF.

How it adds up (as of October 5, 2026)

IVF is one of the clearest examples of peptide drugs working as intended. GnRH antagonists and agonists stop premature ovulation, and hCG or a GnRH agonist triggers final egg maturation. Across 73 trials, antagonists matched the long agonist protocol on live births with less OHSS; an agonist trigger cut OHSS further but lowered live births in fresh transfers, which is why clinics match the trigger to each woman's risk. Kisspeptin has shown promise as a gentler trigger in a small trial but is not approved. Cetrorelix, ganirelix, leuprolide and hCG are approved medicines; the same names sold online as research chemicals are not a substitute for a monitored cycle.

Frequently asked questions

Which is better in IVF, a GnRH agonist or antagonist?

A 2016 Cochrane review of 73 trials found similar live-birth rates (OR 1.02) but less ovarian hyperstimulation syndrome with antagonists (OR 0.61).

Why use a GnRH agonist trigger instead of hCG?

It cuts the risk of OHSS sharply, but in fresh transfers it lowered live births, so clinics use it mainly for high-risk women, often with freeze-all or extra luteal support.

Is kisspeptin used in IVF?

Only in trials. In a phase 2 trial in 60 high-risk women, kisspeptin-54 matured eggs in 95% with no moderate or severe OHSS; it is not approved.

Are cetrorelix and ganirelix approved?

Yes. Cetrotide (cetrorelix) has been FDA-approved since 2000, and ganirelix is marketed as generics, both to prevent premature LH surges during ovarian stimulation.

Does hCG help with weight loss?

No. The US labels for Pregnyl and Novarel state hCG has not been shown to be effective for treating obesity.

Related on Grey Peptides

• Cetrorelix
• Ganirelix
• hCG
• Kisspeptin and fertility

Sources

1. Al-Inany, H. G., et al. (2016). Gonadotrophin-releasing hormone antagonists for assisted reproductive technology. Cochrane Database Syst Rev, 4(4), CD001750. PMID: 27126581
2. Grey Peptides dataset rows for cetrorelix (Cetrotide, NDA 021197, approved August 11, 2000), ganirelix (generic ganirelix acetate ANDAs; label dose 250 mcg once daily until the day of hCG), hCG (Pregnyl, Novarel, generic chorionic gonadotropin; Ovidrel for final follicular maturation; deemed biologics licences March 23, 2020) and leuprolide (Drugs@FDA), read September 28 to October 1, 2026.
3. Toftager, M., et al. (2016). Risk of severe ovarian hyperstimulation syndrome in GnRH antagonist versus GnRH agonist protocol: RCT including 1050 first IVF/ICSI cycles. Hum Reprod, 31(6), 1253-64. PMID: 27060174
4. Youssef, M. A., et al. (2014). Gonadotropin-releasing hormone agonist versus HCG for oocyte triggering in antagonist-assisted reproductive technology. Cochrane Database Syst Rev, 2014(10), CD008046. PMID: 25358904
5. Youssef, M. A., et al. (2016). Recombinant versus urinary human chorionic gonadotrophin for final oocyte maturation triggering in IVF and ICSI cycles. Cochrane Database Syst Rev, 4(4), CD003719. PMID: 27106604
6. Beebeejaun, Y., et al. (2026). Oocyte maturation triggering in high responders in IVF treatment: a systematic review and network meta-analysis. Front Endocrinol (Lausanne), 17, 1669781. PMID: 42006270
7. Abbara, A., et al. (2017). A second dose of kisspeptin-54 improves oocyte maturation in women at high risk of ovarian hyperstimulation syndrome: a Phase 2 randomized controlled trial. Hum Reprod, 32(9), 1915-1924. PMID: 28854728
8. Martins, W. P., et al. (2026). Progestogen-primed ovarian stimulation vs GnRH antagonists and/or agonists for women undergoing assisted reproduction: systematic review and meta-analysis. Ultrasound Obstet Gynecol, 68(2), 188-201. PMID: 42425545
9. Ding, N., et al. (2017). Dual trigger of final oocyte maturation with a combination of GnRH agonist and hCG versus a hCG alone trigger in GnRH antagonist cycle for in vitro fertilization: A Systematic Review and Meta-analysis. Eur J Obstet Gynecol Reprod Biol, 218, 92-98. PMID: 28957685
10. Abbara, A., et al. (2015). Efficacy of Kisspeptin-54 to Trigger Oocyte Maturation in Women at High Risk of Ovarian Hyperstimulation Syndrome (OHSS) During In Vitro Fertilization (IVF) Therapy. J Clin Endocrinol Metab, 100(9), 3322-31. PMID: 26192876
11. Kasum, M., et al. (2017). Kisspeptin as a promising oocyte maturation trigger for in vitro fertilisation in humans. Gynecol Endocrinol, 33(8), 583-587. PMID: 28393578
12. Pregnyl and Novarel (chorionic gonadotropin) US labels, statement that hCG has not been shown to be effective adjunctive therapy for obesity; as recorded in the Grey Peptides hCG dataset row, read October 1, 2026.

Educational information, not medical advice. Each dose in this guide names its source, an approved label or a published study. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose.