Ganirelix

Origin and structure

Ganirelix is a GnRH antagonist: a synthetic relative of gonadotrophin-releasing hormone (GnRH, also called gonadorelin), the ten-amino-acid hormone with which the hypothalamus controls the release of LH and FSH, altered so that it occupies the hormone's receptor without switching it on. Earlier antagonists with arginine at position 6 were potent, but they also made mast cells release histamine, which can cause allergic-type reactions. Chemists at Syntex Research in Palo Alto tried to shield that charge with diethyl-homoarginine, an unnatural amino acid, and in 1992 they described ganirelix as one of two candidates that strongly suppressed ovulation in rats while releasing little histamine in the laboratory. Organon later developed it for IVF under the code Org 37462.

Compared with natural GnRH, six of the ten amino acids are replaced, at positions 1, 2, 3, 6, 8 and 10, five of them by D-amino acids: Ac-D-Nal(2)-D-Phe(4Cl)-D-Pal(3)-Ser-Tyr-D-hArg(Et2)-Leu-hArg(Et2)-Pro-D-Ala-NH2. The formula is C80H113ClN18O13 and the molar mass 1570.4 g/mol as the free base; the medicine contains it as the acetate salt.

Unlike agonists such as triptorelin or leuprorelin, ganirelix is not made as a slow-release depot. It comes as a ready-to-use solution in single-use pre-filled syringes, one injection a day, sold as Orgalutran (Organon) and as generics such as Fyremadel (Sun Pharma) and Ganirelix Gedeon Richter. In the US it was first sold as Antagon; Organon now markets it there simply as Ganirelix Acetate Injection.

How it works

In a normal cycle the pituitary releases LH in pulses, and in mid-cycle a large LH surge makes the leading follicle release its egg. During ovarian stimulation for IVF, FSH injections make many follicles grow at once, and the oestradiol they produce can set off that surge days too early. The eggs may then be released, or the follicles may start turning into a corpus luteum, before they can be collected, and the cycle is lost. Clinics therefore switch off the woman's own LH while the ovaries are stimulated.

Ganirelix does this by competing with natural GnRH for its receptor on the pituitary. Because it blocks rather than stimulates, there is no initial surge as with agonists, and the effect starts within hours: in female volunteers, daily injections lowered LH by up to 74%, FSH by 32% and oestradiol by 25%, with the largest falls 4 to 16 hours after an injection. LH is suppressed more than FSH, the follicles keep growing on the injected FSH, and within two days of the last injection hormone levels are back to normal. That is why ganirelix is started only partway through stimulation, when a surge becomes likely, and stopped at the trigger.

After an injection under the skin about 91% reaches the blood, levels peak within one to two hours and the half-life is about 13 hours; steady levels are reached after two to three days of daily injections. Because the block wears off quickly, the EU label sets a maximum interval between injections. An antagonist also leaves the pituitary able to respond, so in antagonist cycles a single dose of a GnRH agonist such as triptorelin can replace hCG to trigger the final maturation of the eggs, by setting off a short surge of the body's own LH. ESHRE recommends this agonist trigger, combined with freezing all embryos, for women at risk of ovarian hyperstimulation.

Is ganirelix approved?

Yes. Ganirelix is a prescription medicine in the EU and the US, approved for one use: preventing a premature LH surge in women having controlled ovarian stimulation for assisted reproduction, such as IVF and ICSI. The EU label reserves prescribing for specialists experienced in treating infertility.

In the Netherlands it is prescription-only. The Farmacotherapeutisch Kompas lists Orgalutran (Organon), Fyremadel (Sun Pharmaceutical Industries Europe) and generic ganirelix. Since 1 January 2014 fertility hormones, ganirelix and cetrorelix among them, have been paid for as part of hospital care rather than through the outpatient medicines reimbursement list (GVS), so they reach patients through the fertility treatment itself.

We do not sell ganirelix, and this article describes the licensed medicines only. Ganirelix offered as a laboratory chemical has no marketing authorisation, is not made under a medicines licence and may not be used in people.

• EU: Orgalutran was authorised throughout the EU on 17 May 2000 for the prevention of premature LH surges in women undergoing controlled ovarian hyperstimulation for assisted reproduction; in its trials it was combined with recombinant FSH or with the long-acting FSH corifollitropin alfa. The licence is held by N.V. Organon in Oss, the Netherlands. A generic, Ganirelix Gedeon Richter, followed with an EU-wide licence on 15 July 2022, and other generics are licensed country by country.
• Netherlands: besides Orgalutran, the Dutch register holds Fyremadel (Sun Pharmaceutical Industries Europe) and Ganirelix Theramex, licensed on 23 January 2023 through a decentralised procedure led by Portugal.
• US: approved on 29 July 1999 as Antagon (Organon), with priority review, for the inhibition of premature LH surges in women undergoing controlled ovarian hyperstimulation. Fyremadel (Sun Pharma, marketed by Ferring) was approved as a generic on 30 November 2018, and several more generics have followed since 2022.
• Sport: not on the WADA Prohibited List 2026. Its class S2.2.1 bans GnRH and its agonist analogues in men, because they can raise testosterone, but names no antagonists.

What the research shows

Ganirelix reached the market on one dose-finding trial and three randomised phase 3 trials, each comparing it with a long agonist protocol, the standard at the time. Later evidence comes from reviews that pool ganirelix with cetrorelix and from follow-up of the children. The main results:

• Dose-finding (Hum Reprod 1998): 333 women on recombinant FSH were randomised, double-blind, to six daily doses. LH rose to 10 IU/l or more in 16% and 9% at the two lowest doses and in 1.4% at the dose that was later licensed. Higher doses suppressed oestradiol further and did worse: implantation was highest at the licensed dose (21.9%) and lowest at the highest dose, and the lowest and highest dose groups were stopped early on the advice of an independent committee. At the licensed dose 33.8% of women had an ongoing pregnancy per started treatment.
• Against buserelin (European Orgalutran Study Group, Hum Reprod 2000): 730 women randomised 2:1. The median time on a GnRH analogue was 5 days with ganirelix against 26 days with buserelin nasal spray, and less FSH was needed. Ganirelix gave slightly fewer eggs (9.1 against 10.4) and fewer ongoing pregnancies per started treatment (20.3% against 25.7%), but also less ovarian hyperstimulation (2.4% against 5.9%).
• Against leuprolide (North American Ganirelix Study Group, Fertil Steril 2001): 313 women. Eggs per attempt 11.6 against 14.1 and ongoing pregnancies per attempt 30.8% against 36.4%. Moderate injection-site reactions were reported by 11.9% on ganirelix and 24.4% on leuprolide.
• Against triptorelin (European and Middle East Orgalutran Study Group, Hum Reprod 2001): 355 women randomised 2:1. The ganirelix regimen was 17 days shorter on average (9 against 26 days) and used less FSH. Implantation was identical (22.9%), ongoing pregnancies per attempt 31.0% against 33.9%, and local skin reactions half as frequent (11.9% against 24.1%).
• All antagonists against the long agonist protocol (Cochrane review 2016): 73 randomised trials with 12,212 women, ganirelix and cetrorelix together. Live births showed no clear difference (odds ratio 1.02, 95% CI 0.85 to 1.23; 12 trials, 2,303 women), while ovarian hyperstimulation of any grade was less frequent (odds ratio 0.61, 95% CI 0.51 to 0.72; 36 trials, 7,944 women): if 11% develop it on an agonist, 6 to 9% would on an antagonist. Cycles were cancelled more often for a poor response (odds ratio 1.32). The authors rated the evidence moderate quality.
• Children (Hum Reprod 2010): at the university hospital of Brussels, 969 babies born alive after ganirelix were examined in the same way as 963 born after a long buserelin protocol in earlier years. Major malformations were found in 5.0% of fetuses and babies of 26 weeks or more after ganirelix, against 5.4% after the agonist (odds ratio 0.94, 95% CI 0.62 to 1.42).
• Fixed or flexible start (Hum Reprod Update 2023): in a meta-analysis of direct comparisons, starting the antagonist only once the lead follicle reached a set size gave fewer ongoing pregnancies than a fixed start on day 5 or 6 of stimulation (risk ratio 0.76; 6 trials, 907 women; low-certainty evidence). ESHRE's 2025 guideline now conditionally recommends the fixed start.

Ganirelix versus cetrorelix and the long agonist protocol

Before the antagonists arrived, the usual way to prevent an early surge was the long protocol. An agonist such as buserelin, leuprorelin or triptorelin is started in the second half of the previous cycle, first causes a surge and then desensitises the pituitary, and stimulation only begins once that has happened. Ganirelix needs no lead-in because it works within hours: in the phase 3 trials it was given for a median of five days, against almost four weeks of agonist, with fewer injections, less FSH and fewer injection-site reactions. The first trials found slightly fewer eggs and somewhat lower pregnancy rates with ganirelix, but pooled analyses of later trials found no clear difference in live births and less ovarian hyperstimulation. ESHRE has recommended the antagonist protocol over agonist protocols for the general IVF/ICSI population since 2019, a strong recommendation repeated in its 2025 update, citing comparable efficacy and higher safety.

Cetrorelix (Cetrotide, Merck) is the other antagonist licensed for IVF, authorised in the EU on 13 April 1999 and in the US on 11 August 2000. It has the same indication and is also injected once a day, so the differences are mostly practical. Ganirelix comes as a ready-to-use solution in a pre-filled syringe that needs no refrigeration. Cetrotide is a powder that has to be dissolved before each injection and is kept in the fridge, and its EU label asks for the first injection to be given under medical supervision, with 30 minutes of observation. In the Netherlands cetrorelix is now also registered as a ready-to-use solution, Ceziboe.

Direct comparisons are scarce. In a US randomised trial of 185 women (Fertil Steril 2005), neither a single higher-strength injection of cetrorelix nor daily ganirelix let a premature surge through, and pregnancy rates were similar. A 2023 network meta-analysis, three of whose seven authors worked for Merck, found too few data to compare the two on ongoing pregnancy. The largest comparison is observational: at a Chinese centre, 2,365 women on cetrorelix and 7,059 on ganirelix, matched for age, BMI and ovarian reserve, had similar live-birth rates (47.2% against 49.4%), while the ganirelix group more often had an LH rise to 10 IU/l or more on the trigger day (7.6% against 4.9%) and ovarian hyperstimulation (1.1% against 0.4%). A retrospective study cannot rule out that the groups differed in other ways.

Side effects and warnings

Ganirelix itself causes few side effects; most problems in a treatment cycle come from the stimulation. The EU product information lists a skin reaction at the injection site, mostly redness with or without swelling, as very common: in the trials 12% reported a moderate or severe reaction at least once per cycle, against 25% with an agonist injected under the skin, and the reactions usually faded within four hours. Headache, nausea and malaise are uncommon. In the two controlled trials on which the US label rests (794 women), the events recorded most often from the first dose until pregnancy was confirmed, regardless of cause, were gynaecological abdominal pain (4.8%), loss of the pregnancy (3.7%), headache (3.0%), ovarian hyperstimulation syndrome (2.4%) and vaginal bleeding (1.8%). The US label also notes a raised neutrophil count in 11.9% of trial participants, of unknown significance.

Allergic reactions, including anaphylaxis, angioedema and hives, have been reported after marketing, sometimes with the very first dose; the EU product information rates them as very rare. The labels advise special care in women with an active allergic condition and advise against ganirelix in severe allergic conditions, for which there is no experience. Ovarian hyperstimulation syndrome is a risk of the gonadotrophin stimulation rather than of ganirelix itself, and ectopic pregnancy is more common after IVF in general. In studies of more than 1,000 newborns, congenital malformations after ganirelix were as frequent as after an agonist.

Ganirelix is contraindicated in pregnancy and breastfeeding, in women allergic to GnRH or its analogues, and in moderate or severe kidney or liver impairment. In rats and rabbits the highest doses caused loss of litters, without malformations. Safety and efficacy have not been established in women weighing under 50 kg or over 90 kg, and LH rises tended to be more frequent above 80 kg, without a visible effect on the outcome. No interaction studies have been done, and interactions with drugs that release histamine cannot be excluded.

Frequently asked questions

What does ganirelix do during IVF?

It stops the body's own LH surge from arriving too early. During stimulation many follicles grow at once, and their rising oestradiol can set off a surge before the eggs are collected, which can release the eggs or change the follicles too soon and cost the cycle. Ganirelix blocks the GnRH receptor in the pituitary within hours, so it is added partway through stimulation and stopped at the trigger. In the European phase 3 trial, as summarised on the US label, a premature surge occurred in fewer than 1% of the 463 women treated with ganirelix.

What are the side effects of ganirelix (Orgalutran)?

The most common is redness, sometimes with swelling, where the needle went in: in the trials 12% reported a moderate or severe reaction at least once per cycle, half the rate seen with an injected agonist, and it usually fades within four hours. Headache, nausea and malaise are uncommon. Allergic reactions, including anaphylaxis, are very rare but can occur with the first dose. Ovarian hyperstimulation syndrome can occur in any stimulation cycle; it is a risk of the gonadotrophins rather than of ganirelix itself.

What is the difference between ganirelix and cetrorelix (Orgalutran and Cetrotide)?

Both are GnRH antagonists licensed for the same purpose and injected once a day under the skin. The differences are mostly practical: ganirelix comes ready to use in a pre-filled syringe that needs no fridge, whereas Cetrotide is a powder that is dissolved before injection, is kept in the fridge, and under its EU label is first injected under medical supervision; Ceziboe now offers cetrorelix ready to use as well. Direct comparisons are few: one US randomised trial of 185 women found no premature surges and similar pregnancy rates with either, and a 2023 network meta-analysis found too few data to rank them.

Is the antagonist protocol better than the long protocol?

For most women, European guidance says yes. ESHRE's 2025 guideline recommends the GnRH antagonist protocol over agonist protocols for the general IVF/ICSI population, citing comparable efficacy and higher safety. The 2016 Cochrane review of 73 trials found no clear difference in live births and less ovarian hyperstimulation (odds ratio 0.61). Antagonist cycles are also much shorter, and they allow an agonist trigger instead of hCG, which lowers that risk further. Long agonist protocols are still used in some situations, and the choice belongs to the clinic.

Is ganirelix reimbursed in the Netherlands?

Not through the pharmacy. Since 1 January 2014 fertility hormones, ganirelix and cetrorelix among them, have been part of hospital care rather than the outpatient medicines reimbursement list (GVS), so their cost is covered through the fertility treatment the hospital provides. Orgalutran, Fyremadel and generic ganirelix are prescription-only and are prescribed by fertility specialists. Ganirelix sold as a laboratory chemical is not a medicine and may not be used in people.

Is ganirelix banned in sport?

No. The WADA Prohibited List 2026 bans, in men and at all times, GnRH and its agonist analogues such as triptorelin and leuprorelin, in class S2.2.1, testosterone-stimulating peptides in males. GnRH antagonists such as ganirelix lower testosterone instead and are not named on the list, and the 2027 list, published in September 2026, announced no change to this class. The Dutch translation of the list by the Dopingautoriteit carries the same text.

Storage and handling

Store according to product-label instructions and batch handling notes. Lyophilized research peptides are commonly kept dry, protected from light, and handled with clean laboratory technique.

References

Checked against these sources on 28 October 2026.

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