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Cetrorelix

Also known as Cetrotide, cetrorelix acetate, SB-75

GnRH antagonist used in IVF: blocks the receptor immediately, suppressing LH without the flare seen with agonists.

clinically proven Hormonal & sexual approved drug

At a glance

Category
Hormonal & sexual
Status
approved drug
Route
subcutaneous, into the lower abdomen
Half-life
roughly 5 hours after a single 0.25 mg subcutaneous dose and around 63 hours after a 3 mg dose; subcutaneous bioavailability approximately 85% 1
Onset
LH suppression within hours of the first injection; no initial stimulation
Molecular weight
1431.06 g/mol calculated as the anhydrous free base 1; supplied as the acetate
Sequence
Ac-D-Nal(2)-D-Cpa-D-Pal(3)-Ser-Tyr-D-Cit-Leu-Arg-Pro-D-Ala-NH2 (a decapeptide with five substitutions relative to native GnRH)

Approved in the US and EU as Cetrotide, indicated for the inhibition of premature LH surges in women undergoing controlled ovarian stimulation 1. It is a specialist fertility product intended to be prescribed by physicians experienced in fertility treatment. The 3 mg single-dose presentation has been discontinued in several markets, leaving the 0.25 mg daily form.

Doping status: Not listed by WADA

Checked against the WADA 2026 Prohibited List. For a specific product and country, Global DRO is the lookup athletes are expected to use — the List names substances, not brand names.

Mechanism of action

Cetrorelix is a synthetic decapeptide that competes with endogenous GnRH for binding to receptors on pituitary gonadotroph cells. Because it occupies the receptor without activating it, LH and FSH secretion fall from the first dose. The effect is dose-dependent and reverses within a day or so of stopping, as the drug clears and endogenous GnRH regains access to the receptor.

The contrast with GnRH agonists is the single most important point about this drug class and it is routinely stated backwards. Triptorelin, leuprolide and nafarelin are agonists: they activate the receptor, so the first one to two weeks of treatment produce a surge in LH, FSH and gonadal steroids — the flare — and only after receptor downregulation does suppression follow. Cetrorelix and ganirelix are antagonists: they never activate the receptor, so there is no flare, suppression begins within hours, and there is no downregulation period to wait through. In practical terms an agonist needs to be started weeks before the effect is wanted, whereas an antagonist can be started mid-cycle at the moment suppression becomes necessary.

That timing property is precisely what makes antagonists useful in IVF. In a stimulation cycle, follicles are grown with exogenous FSH, and the risk is that rising oestradiol triggers a premature endogenous LH surge that causes ovulation before oocytes can be retrieved. Cetrorelix is introduced part-way through stimulation — either as 0.25 mg daily from around day 5-6, or as a single 3 mg dose once oestradiol reaches a threshold 1 — and holds the pituitary silent until the clinician administers a deliberate trigger.

A second consequence follows from the absence of downregulation. Because the pituitary is not desensitised, an antagonist cycle can be triggered with a GnRH agonist instead of hCG, using the pituitary's own LH reserve. This produces a much shorter LH exposure than hCG and substantially reduces the risk of ovarian hyperstimulation syndrome. This option simply does not exist in an agonist long protocol.

Cetrorelix has also been investigated at higher doses for benign prostatic hyperplasia and endometriosis, where prolonged administration produces hypogonadism without a flare, but these applications are not licensed.

What the research shows

Cetrorelix is a licensed drug with a well-defined evidence base in assisted reproduction. Randomised trials and a large Cochrane synthesis show that antagonist protocols achieve live birth rates comparable to the agonist long protocol while substantially reducing ovarian hyperstimulation syndrome. Evidence outside fertility medicine is limited and does not support use.

Research in humans

Registrational trials established suppression of premature LH surges and defined the 0.25 mg daily and 3 mg single-dose regimens 1. The 2016 Cochrane review by Al-Inany and colleagues, pooling a large number of randomised trials of GnRH antagonists versus the long agonist protocol, found moderate-quality evidence of a substantial reduction in OHSS without a reduction in live birth 2. Antagonist protocols have since become the default in many fertility units, particularly for women at high risk of OHSS.

Animal and lab research

Preclinical work characterised competitive receptor binding, the absence of a flare response and the rapid reversibility of suppression. Higher-dose animal studies explored prostate and endometrial applications. These support mechanism rather than clinical protocol.

Caveats. The trial population is women undergoing IVF, so conclusions do not transfer to other settings. The 3 mg single-dose regimen has less accumulated data than daily dosing and has been withdrawn in some markets. There are few direct comparisons between cetrorelix and ganirelix, which are generally treated as equivalent within the class. No evidence supports use for hormonal manipulation outside a supervised fertility protocol.

What it is used for

  • Inhibition of premature LH surges in women undergoing controlled ovarian stimulation for assisted reproduction 1
  • Enabling a GnRH agonist trigger instead of hCG, as an OHSS-reduction strategy
  • Investigational and unlicensed: benign prostatic hyperplasia, endometriosis, uterine fibroids and hormone-sensitive tumours

Dosing

Dose
0.25 mg per injection (multiple-dose regimen) or 3 mg as a single dose
Frequency
0.25 mg once daily; or a single 3 mg dose
Route
subcutaneous, into the lower abdominal wall, rotating sites
Duration
Multiple-dose: from stimulation day 5 or 6 until the day of the ovulation trigger, typically 4-7 days. Single-dose: one injection, with 0.25 mg daily added from 96 hours later if the trigger has not yet been given
  • The multiple-dose regimen may be started by stimulation day, usually day 5 or 6, or by follicular criteria, whichever the unit's protocol specifies. Timing is directed by ultrasound and oestradiol monitoring, not by a fixed calendar.
  • The single 3 mg dose is given when adequate oestradiol levels are reached, typically around stimulation day 7. Its longer half-life covers roughly four days.
  • Injection timing within the day should be consistent — most units specify either morning or evening dosing and keep it constant across the cycle.
  • There is no legitimate dosing protocol for cetrorelix outside a monitored fertility cycle. Doses circulating for other purposes are not derived from clinical evidence.

These figures describe what the literature and published protocols report. They are not advice and not a dosing instruction.

Protocols

Cetrotide 0.25 mg, multiple-dose regimen (controlled ovarian stimulation)

approved product information

Source: CETROTIDE (cetrorelix acetate) prescribing information, DailyMed

Stimulation day 5 or 6, or when follicular criteria are met0.25 mg subcutaneously into the lower abdominal wall, once daily
Each day thereafter, up to and including the day of the ovulation trigger0.25 mg once daily, usually 4-7 doses in total

An antagonist blocks the GnRH receptor directly, so LH is suppressed within hours and there is no initial flare - which is why it can be started mid-stimulation rather than a cycle ahead. The start day is set by ultrasound and oestradiol monitoring, and the time of day is kept constant across the cycle.

Cetrotide 3 mg, single-dose regimen (controlled ovarian stimulation)

approved product information

Source: CETROTIDE (cetrorelix acetate) prescribing information, DailyMed

When adequate oestradiol levels are reached, usually around stimulation day 73 mg subcutaneously, once
From 96 hours after the 3 mg dose, if the trigger has not yet been given0.25 mg once daily until and including the day of the trigger

The single 3 mg dose covers roughly four days; the 0.25 mg top-ups exist for cycles that run longer than that. The two regimens are alternatives - they are not combined from the start.

Schedules are reproduced as their source states them. Units, IU and milligrams explains why the figures are not interchangeable between products.

Reconstitution

Vial sizes
0.25 mg, 3 mg (discontinued in some markets)
Solvent
Sterile water for injection, supplied in a prefilled syringe with the kit. Bacteriostatic water is not used
Storage
Store the 0.25 mg kit refrigerated at 2-8 °C in the original carton, protected from light. Reconstitute immediately before use and inject at once — the product contains no preservative.

Worked example

The kit contains a vial of lyophilised powder and a prefilled syringe of 1 mL sterile water. The entire reconstituted volume is one dose, so there is no concentration calculation; the syringe is supplied with a larger reconstitution needle and a finer 27-gauge needle for the subcutaneous injection.

Dissolve by gentle swirling; do not shake. Cetrorelix is formulated with mannitol as a bulking agent, and patients with mannitol hypersensitivity should not receive it.

Work it out for Cetrorelix

Safety

Side effects

  • Ovarian hyperstimulation syndrome, reported in around 3.5% of patients in trials 1 — largely a consequence of the stimulation cycle rather than of cetrorelix itself, and lower in antagonist than in agonist protocols
  • Local injection site reactions: redness, swelling, itching, usually transient and mild
  • Nausea, reported in roughly 1.3% 1
  • Headache, roughly 1.1% 1
  • Abdominal pain and bloating
  • Rare hypersensitivity reactions, including reports of anaphylactoid reactions
  • The pronounced hypo-oestrogenic effects seen with agonists — hot flushes, bone loss — are not a feature of short antagonist courses, because exposure lasts days rather than months

Do not use if

  • Hypersensitivity to cetrorelix, to extrinsic peptide hormones, to mannitol, or to GnRH or any other GnRH analogue 1
  • Known or suspected pregnancy 1
  • Breastfeeding 1
  • Severe renal impairment 1
  • Postmenopausal women
  • Should be prescribed only by physicians experienced in fertility treatment, with ultrasound and hormone monitoring available

Interactions

No formal interaction studies have been conducted with cetrorelix and no clinically relevant drug interactions are established 1. Cetrorelix is a peptide, is not a cytochrome P450 substrate or inhibitor, and the relevant interactions are pharmacodynamic: concurrent GnRH agonists work against it, and it is used deliberately in combination with gonadotropins, where the combined effect on the ovary determines OHSS risk. Exogenous oestrogens or progestogens interfere with the monitoring that guides its timing.

Sources