Gonadorelin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	IM or SC	2-5 mcg/kg (for ovulation induction); 50-100 mcg per dog for GnRH stimulation test	Single dose for ovulation induction; for stimulation test, one dose	Duration: Single administration; may be repeated in 2-3 days if no response Notes: For ovulation induction, administer when a mature follicle is detected (typically 2-4 days after estrus onset). For GnRH test, measure LH before and 30-60 minutes after administration.
Cat	IM or SC	25-50 mcg per cat (for ovulation induction); 25 mcg per cat for GnRH test	Single dose	Duration: Single administration Notes: Administer during estrus when follicles are mature. Ovulation occurs 24-48 hours post-injection.
Horse	IM or IV	1-2 mg per horse (for ovulation induction); 0.5-1 mg for GnRH test	Single dose	Duration: Single administration Notes: For mares, administer when a follicle ≥35 mm is present. Ovulation typically occurs 24-48 hours later.
Cattle	IM	100-200 mcg per cow (for ovulation induction or synchronization); 100 mcg for GnRH test	Single dose or as part of protocol (e.g., Ovsynch: GnRH at day 0, PGF2α at day 7, GnRH at day 9)	Duration: Single or multiple doses depending on protocol Notes: In Ovsynch, administer GnRH at the start and 48 hours after PGF2α. For follicular cysts, 100-200 mcg IM.
Sheep/Goat	IM	50-100 mcg per ewe/doe	Single dose	Duration: Single administration Notes: Used to induce ovulation in synchronized estrus protocols.
Rabbit	IM or SC	0.5-1 mcg/kg	Single dose	Duration: Single administration Notes: For ovulation induction in breeding programs.

Clinical Indications & Species Uses

General Indications

Diagnostic agent for evaluating pituitary gonadotropic function
Therapeutic agent for reproductive disorders involving gonadotropin deficiency

Dog (Canine)

Treatment of ovarian follicular cysts
Induction of ovulation in bitches (timing of mating)
Treatment of cryptorchidism (in combination with hCG)
Diagnostic testing of pituitary function (GnRH stimulation test)

Cat (Feline)

Induction of ovulation in queens (for controlled breeding)
Treatment of ovarian follicular cysts (rare)
Diagnostic testing of pituitary function

Horse (Equine)

Induction of ovulation in mares (timing of breeding)
Treatment of persistent anestrus (with progesterone priming)
Treatment of ovarian follicular cysts
Diagnostic testing of pituitary function

Cattle (Bovine)

Induction of ovulation in cows (synchronization protocols, e.g., Ovsynch)
Treatment of ovarian follicular cysts
Treatment of silent estrus
Improvement of conception rates in timed AI protocols

Small Ruminants (Sheep / Goat)

Induction of ovulation in ewes and does (during breeding season)
Treatment of follicular cysts (rare)
Synchronization of estrus in protocols

Rabbit & Small Mammals

Induction of ovulation (rabbits are induced ovulators)
Used in research settings for reproductive studies

Exotic & Other Species

Induction of ovulation in induced ovulators (e.g., ferrets, camels)
Reproductive management in zoo and wildlife species (e.g., elephants, rhinoceros) under specialist guidance

Pharmacology & Mechanism of Action

Drug Class: Gonadotropin-releasing hormone (GnRH) analog | Pharmacological Group: Hypothalamic hormone / Gonadotropin-releasing hormone

Mechanism of Action: Gonadorelin is a synthetic decapeptide identical to endogenous GnRH. It binds to GnRH receptors on the anterior pituitary gonadotroph cells, stimulating the synthesis and release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In males, LH stimulates Leydig cells to produce testosterone; in females, LH triggers ovulation and supports luteal function. The pulsatile release of endogenous GnRH is essential for normal reproductive function; continuous or high-dose administration can paradoxically downregulate GnRH receptors, leading to suppression of gonadotropins and sex steroids.

Pharmacodynamics: Gonadorelin induces a rapid, dose-dependent release of LH and FSH. In females, a single dose can induce ovulation in mature follicles, with ovulation occurring approximately 24-48 hours post-administration. In males, it stimulates testosterone production. The response is influenced by the stage of the estrous cycle, species, and dose. Chronic administration can cause desensitization of the pituitary, reducing LH/FSH secretion, which is used therapeutically for contraception or treatment of hormone-dependent conditions.

⚡ Pharmacokinetics Summary

Absorption: Gonadorelin is rapidly absorbed after subcutaneous or intramuscular injection. Oral bioavailability is negligible due to gastrointestinal degradation. Intranasal and intravaginal routes have been explored but are not commonly used in veterinary medicine.

Distribution: Distributes widely into tissues, with highest concentrations in the pituitary gland. It crosses the blood-brain barrier poorly. Volume of distribution is approximately 0.2-0.3 L/kg in dogs.

Metabolism: Rapidly metabolized by peptidases in plasma and tissues, primarily in the liver and kidneys. The half-life is very short, typically 2-5 minutes in most species.

Excretion: Metabolites are excreted primarily in urine, with a small amount in feces. Less than 5% of the parent compound is excreted unchanged.

Half-Life: 2-5 minutes in dogs, cats, horses, and cattle; slightly longer in some species (up to 10 minutes).

Bioavailability: Subcutaneous and intramuscular routes provide near-complete bioavailability (approximately 100%). Oral bioavailability is negligible.

Protein Binding: Minimal protein binding (<10%).

Available Formulations & Strengths

Injectable Solution 50 mcg/mL, 100 mcg/mL, 200 mcg/mL (IM, SC, IV)

Lyophilized Powder for Injection Vials containing 50 mcg, 100 mcg, 500 mcg, 1 mg, 2 mg (IM, SC, IV)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to gonadorelin or any component of the formulation
Pregnancy (unless specifically intended for pregnancy termination protocols, which is not standard)
Use in animals with pituitary tumors (may cause acute tumor swelling)
Do not use in animals with undiagnosed reproductive tract abnormalities without proper diagnostic workup

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal impairment (dose adjustment may be necessary, though not well-studied)
May cause transient ovarian hyperstimulation in some species; monitor for multiple ovulations
In food animals, observe withdrawal times; use only in accordance with label or veterinary supervision
For diagnostic use, interpret results in conjunction with clinical findings and other endocrine tests
Repeated use may lead to antibody formation and reduced efficacy
In mares, use only when a mature follicle is present to avoid multiple ovulations
In cattle, use as part of a complete reproductive management program

Adverse Effects & Reactions

Common:

Injection site reactions (pain, swelling)
Transient behavioral changes (increased libido or estrus behavior)
Mild gastrointestinal upset (rare)

Serious / Severe:

Ovarian hyperstimulation syndrome (rare, but possible with high doses)
Anaphylactic reactions (rare)
Tumor flare in animals with pituitary tumors

Rare:

Allergic reactions
Local abscess at injection site
Temporary suppression of gonadotropins with chronic use

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Prostaglandin F2α (PGF2α)	Used concurrently in synchronization protocols; PGF2α induces luteolysis, allowing GnRH to induce ovulation in a new follicular wave.	Moderate
Progestins (e.g., altrenogest, medroxyprogesterone)	Progestins can suppress endogenous GnRH; concurrent use may reduce the efficacy of exogenous gonadorelin.	Moderate
Corticosteroids	May suppress the pituitary response to GnRH, reducing LH release.	Mild
GnRH antagonists (e.g., cetrorelix)	Competitive antagonism; concurrent use would negate the effects of gonadorelin.	High
Human chorionic gonadotropin (hCG)	May have additive effects on ovulation induction; used together in some protocols.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Ovarian hyperstimulation (multiple ovulations, cystic ovaries)
Prolonged estrus or abnormal reproductive cycles
Injection site reactions
In rare cases, anaphylactoid reactions

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Monitor reproductive status and manage any ovarian hyperstimulation (e.g., with fluid therapy and anti-inflammatory drugs if needed). In cases of anaphylaxis, administer epinephrine and supportive care. Withdrawal times may be extended; consult regulatory guidelines.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Gonadorelin is a naturally occurring peptide and is rapidly metabolized; zero withdrawal times are generally assigned. However, always follow label instructions and local regulations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at 2-8°C (refrigerate); do not freeze.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light. Reconstituted solutions should be used immediately or within 24 hours if stored refrigerated. Discard unused portions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for veterinary use in cattle (e.g., Cystorelin, Factrel) and horses (e.g., Gonadorelin injection).

Extra-Label (Off-Label) Use: In the US, gonadorelin is approved for use in cattle and horses. Extra-label use in other species is permitted under AMDUCA with a valid veterinarian-client-patient relationship and appropriate withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Gonadorelin is a valuable tool in reproductive management. In cattle, it is a cornerstone of timed AI protocols like Ovsynch. In mares, it is used to precisely time ovulation for breeding. In small animals, it is used for ovulation induction and diagnostic testing. Because of its short half-life, it must be administered close to the time of desired effect. For best results, use in conjunction with ultrasonography or other methods to confirm follicular maturity. In food animals, adhere to withdrawal times and follow label directions. In exotic species, consult a specialist for dosing and administration.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)