FDA-approved
Sexual & reproductive
Ganirelix
Ganirelix is an FDA-approved GnRH antagonist peptide that prevents early ovulation during IVF. Learn how it works, what it is used for, and its side effects.
- Reviewed by
- William Maish, MD MBA MPH, Clinical Product Lead
- Published
- October 5, 2026
- Last updated
- October 5, 2026
Key takeaway
Ganirelix (originally approved as Antagon) is a synthetic decapeptide that blocks the gonadotropin-releasing hormone receptor, used during fertility treatment to stop an egg from being released too early. Given once daily during ovarian stimulation, it prevents the premature luteinizing hormone surge that could otherwise disrupt an IVF cycle. It has been FDA-approved in the U.S. since 1999 and is given as a small daily subcutaneous injection.
At a glance
| What it is | A GnRH receptor antagonist peptide used in assisted reproduction |
| Approved for | Preventing premature luteinizing hormone surges during controlled ovarian stimulation (IVF) |
| Key fact | In a phase 3 trial it produced pregnancy rates comparable to a longer leuprolide protocol with fewer injections |
| Route | Subcutaneous injection, once daily during stimulation |
| Evidence strength | FDA approved |
What is ganirelix?
Ganirelix, known generically as ganirelix acetate and originally approved under the brand Antagon (marketed in some regions as Orgalutran), is a synthetic ten-amino-acid peptide that antagonizes the gonadotropin-releasing hormone (GnRH) receptor. It is used specifically in assisted reproductive technology to control the timing of ovulation. It was developed under the code Org 37462 and approved in the United States in 1999. In the U.S. it is now sold as ganirelix acetate injection and under the brand Fyremadel1.
How does ganirelix work?
Ganirelix competitively blocks GnRH receptors on the pituitary, rapidly suppressing the release of luteinizing hormone and follicle-stimulating hormone. During an IVF cycle, the ovaries are stimulated with injectable hormones to grow multiple follicles; if the body's own luteinizing hormone surges before those follicles are ready, ovulation can happen too early and the cycle can be lost. Giving ganirelix during stimulation holds that surge in check so eggs can be retrieved at the planned time.
Because it is an antagonist, its effect is immediate and reversible, which allows a shorter, more flexible protocol than the older agonist-based approach that first had to suppress the pituitary over days. In a North American phase 3 trial, ganirelix compared with a long leuprolide protocol as follows:
| Outcome | Ganirelix vs long leuprolide protocol |
| Ongoing pregnancy rates | Comparable2 |
| Number of injections | Fewer |
| Injection-site reactions | Fewer |
Is it FDA-approved?
Yes. Ganirelix, developed by Organon, was approved3 by the FDA in 1999. It is approved as a 0.25 mg once-daily subcutaneous injection for use during controlled ovarian stimulation. The Antagon brand name is no longer used in the U.S., where the product is now sold as ganirelix acetate injection and as Fyremadel.
What is ganirelix prescribed for?
Ganirelix is prescribed to inhibit premature luteinizing hormone surges in women undergoing controlled ovarian stimulation as part of assisted reproduction. It is used only during the stimulation phase of a treatment cycle, not on an ongoing basis.
What are the side effects?
According to the prescribing information4, common side effects include:
- Injection-site reactions
- Headache
- Nausea
- Abdominal or pelvic discomfort
More serious considerations include:
- Ovarian hyperstimulation syndrome, a risk of the overall stimulation cycle rather than of ganirelix specifically
- Hypersensitivity reactions
Ganirelix is contraindicated in pregnancy.
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Sources: FDA prescribing information for ganirelix acetate injection; the North American phase 3 trial (Fluker et al., Fertility and Sterility, 2001, DOI: 10.1016/s0015-0282(00)01638-12), indexed on PubMed. This document is educational and does not constitute medical advice.