Gonadotrophin-releasing hormone (GnRH) analogues are the clearest example in peptide pharmacology of a class that has crossed fully from experimental molecule to licensed medicine. Seven compounds in the Peptide Data register — gonadorelin, buserelin, goserelin, leuprolide, triptorelin, cetrorelix and degarelix — belong to this class, and unlike most entries in this reference every one of them is a prescription-only medicine (POM) in the UK, not a research-use-only material. That distinction matters more than any single pharmacological detail. This explainer sets out how the class works, why the labels "agonist" and "antagonist" describe behaviour that converges on the same endpoint, and where the UK legal boundary sits.

The hypothalamic–pituitary–gonadal axis

GnRH is a ten-amino-acid peptide released by a small population of hypothalamic neurons in pulses, not continuously. Those pulses reach the anterior pituitary, where they bind the GnRH receptor and drive release of luteinising hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH in turn govern gonadal steroid production — testosterone and oestrogen — and gametogenesis. The whole chain is the hypothalamic–pituitary–gonadal (HPG) axis, and it is unusual among endocrine systems in that its signalling depends as much on the rhythm of the pulse as on the amount of hormone released.

That pulse dependence is what makes the axis pharmacologically tractable. A peptide that mimics GnRH can, depending on how it is delivered, either amplify the signal or silence it — which is why a single receptor can be targeted to opposite clinical ends.

The paradox at the centre of the class: why agonists suppress

Native GnRH has a half-life of only a few minutes, because peptidases cleave it rapidly. The therapeutically useful analogues are structurally modified at the cleavage sites to resist that degradation, extending their activity from minutes to hours or days. The first generation of these drugs are receptor agonists: they bind and activate the GnRH receptor.

An agonist given as a single pulse would raise gonadotrophin release. Given continuously — the way depot preparations are dosed — the effect inverts. Continuous occupation of the receptor produces an initial surge in LH and FSH, the so-called "flare effect", followed by receptor desensitisation and down-regulation. Within roughly one to three weeks of sustained exposure the pituitary becomes refractory even to its own endogenous GnRH, and gonadotrophin output collapses. The consequence is a profound fall in testosterone or oestrogen — a reversible, pharmacological equivalent of surgical castration or menopause.

This is the paradox that names the class: an agonist — an activator — is used clinically to produce sustained suppression. The flare is not incidental. During the first weeks of treatment the transient rise in sex steroids can temporarily worsen hormone-sensitive disease; in advanced prostate cancer a tumour "flare" can occasionally trigger spinal-cord compression or urinary obstruction, which is why initiation is clinically supervised and sometimes covered with an anti-androgen.

Antagonists: the same endpoint without the flare

The second generation of peptide analogues takes the opposite receptor route. Compounds such as cetrorelix and degarelix are competitive antagonists: they occupy the GnRH receptor without activating it, blocking endogenous GnRH from binding. Because they do not stimulate the receptor, there is no flare, and suppression begins within about 24 hours rather than over weeks. The clinical response is faster and is also quicker to reverse on stopping.

The structural work that made peptide antagonists viable centred on substitutions at positions 6 and 8 of the decapeptide backbone, which removed residual agonist activity that had limited early attempts. The same design logic later produced orally available non-peptide antagonists for the same receptor — a development that has broadened the class beyond injectable peptides and that sits outside the scope of this peptide-focused reference.

Where the class is licensed in the UK

Every compound in this class on the Peptide Data register is a licensed medicine in the UK, and the licensed indications fall into three broad groups:

  • Hormone-sensitive cancer, chiefly prostate cancer, where androgen deprivation is a mainstay of treatment; goserelin is also licensed in breast cancer.
  • Gynaecological conditions driven by oestrogen, including endometriosis and uterine fibroids, and for pre-surgical shrinking of fibroids.
  • Reproductive medicine and developmental endocrinology, including pituitary down-regulation in assisted reproduction and the delay of central precocious puberty in children.

A single class therefore reaches across oncology, gynaecology and fertility from the same receptor mechanism. UK prescribing guidance for prostate cancer treats the various agonists as broadly similar in effectiveness, with differences between agents sitting mainly in formulation and dosing interval rather than in efficacy.

Why the HPG axis matters outside reproductive medicine

Because the axis controls sex-steroid production, anything that suppresses it has effects well beyond the target indication. Sustained hypogonadism causes loss of bone mineral density and increased fracture risk; men commonly experience hot flushes, fatigue and loss of libido; and the effects on mood and metabolic markers are the subject of ongoing study. This is why long-term use is monitored rather than assumed benign, and why "add-back" hormone therapy is sometimes used alongside GnRH suppression in non-oncological settings. None of this makes the class unsafe in its licensed hands — it makes it a class that requires endocrine supervision.

The UK legal position

Here the GnRH analogues sit apart from most of this reference. They are prescription-only medicines under the Human Medicines Regulations 2012, supplied against a prescription from a doctor and dispensed through a pharmacy. They are not "research-use-only" materials, and there is no lawful route to their supply as research chemicals: possession of a POM without a prescription, and supply of a POM other than by a registered pharmacy against a prescription, are offences under UK medicines law. A laboratory that genuinely needs a GnRH analogue as an assay standard or reference material must source it through the legitimate chemical-supply chain, not through a market that also sells unlicensed research peptides.

The practical point for readers is the inverse of the usual one. For most compounds in this reference the question is whether a research material has drifted into the grey market; for the GnRH analogues the question is the opposite — these are mainstream, well-characterised medicines, and any listing that offers them alongside "research use only" peptides is either mislabelling them or handling them outside the licence.

What the evidence shows — and what it does not

The GnRH analogues are among the most extensively documented peptide drug classes in medicine, with four decades of use and a literature spanning thousands of studies. That evidence base supports their licensed indications; it does not translate into support for any non-licensed use, and it says nothing about the safety or handling of materials from unregulated sources. Peptide Data describes research and pharmacology only and does not provide consumption, dosing or self-administration guidance. All compounds discussed here are prescription-only medicines in the UK and are outside research-use-only framing.

This article is AI-researched and editorially reviewed. It is provided for research and educational purposes only and is not medical advice. Research peptides are not licensed for human consumption in the UK.