FDA-approved
Sexual & reproductive
Degarelix
Degarelix is an FDA-approved GnRH antagonist for advanced prostate cancer that lowers testosterone quickly. Learn how it works 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
Degarelix (sold as Firmagon) is a synthetic decapeptide that blocks the gonadotropin-releasing hormone receptor, dropping testosterone to castrate levels within a few days and without the initial surge seen with older hormone drugs. It is used to treat advanced prostate cancer and lowers testosterone to castrate levels faster than the agonist leuprolide. It has been FDA-approved in the U.S. since 2008 and is given as a monthly subcutaneous injection.
At a glance
| What it is | A GnRH receptor antagonist peptide that suppresses testosterone without a flare |
| Approved for | Advanced prostate cancer |
| Key fact | In a phase 3 trial it matched leuprolide for maintaining castrate testosterone over a year and lowered testosterone faster |
| Route | Subcutaneous injection (a loading dose, then monthly) |
| Evidence strength | FDA approved |
What is degarelix?
Degarelix, sold under the brand Firmagon, is a synthetic ten-amino-acid peptide that acts as an antagonist at the gonadotropin-releasing hormone (GnRH) receptor. Where agonists such as leuprolide and goserelin first stimulate the pituitary before suppressing it, degarelix blocks the receptor directly from the first dose. It was developed by Ferring under the code FE200486 and approved in the United States in 2008.
How does degarelix work?
Degarelix binds reversibly and competitively to GnRH receptors on the pituitary, immediately blocking the hormone signal that normally drives release of luteinizing hormone and follicle-stimulating hormone. Those gonadotropins fall within hours, and testosterone drops to castrate levels within a few days. Because it is an antagonist rather than an agonist, it causes no initial testosterone surge, so there is no need for the anti-androgen cover used to blunt the "flare" with agonist drugs.
In advanced prostate cancer, removing testosterone deprives hormone-sensitive tumor cells of the signal they depend on. In the phase 3 CS21 trial, degarelix was non-inferior1 to leuprolide for keeping testosterone at or below castrate levels over 12 months, and it suppressed testosterone and PSA more quickly in the first weeks. That rapid onset without a flare is the main clinical distinction of the drug.
Is it FDA-approved?
Yes. Degarelix was approved2 by the FDA in 2008 for advanced prostate cancer. It is given as a subcutaneous injection, starting with a loading dose followed by a maintenance dose every 28 days.
What is degarelix prescribed for?
Degarelix is prescribed for the treatment of advanced (hormone-sensitive) prostate cancer. Its rapid, flare-free testosterone suppression makes it useful when a fast drop in testosterone is desirable, such as in men with symptomatic or high-burden disease.
What are the side effects?
According to the prescribing information, common side effects include:
- Injection-site reactions such as pain, redness, and swelling (common with the loading dose)
- Hot flashes
- Weight gain and fatigue
- Increased liver enzymes
More serious considerations include:
- QT-interval prolongation, so caution is advised with other QT-prolonging drugs or electrolyte abnormalities
- Hypersensitivity reactions
- Reduced bone density with long-term androgen deprivation
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Sources: FDA prescribing information for Firmagon; the CS21 phase 3 trial (Klotz et al., BJU International, 2008, DOI: 10.1111/j.1464-410X.2008.08183.x1), indexed on PubMed. This document is educational and does not constitute medical advice.