GnRF Analogue–Diphtheria Toxoid Conjugate
The pig vaccine that makes the animal's own antibodies switch off its reproductive hormones, what the studies show, and why it is not for people
This product joins a synthetic analogue of gonadotropin-releasing factor (GnRF, also called GnRH) to a carrier protein, diphtheria toxoid, so that the immune system makes antibodies against the animal's own GnRH. Two injections a few weeks apart shut down the testes or ovaries for a period of weeks to months. It is sold as Improvac in Europe and Improvest in the US to replace surgical castration of male pigs and to stop gilts coming into heat. No study in people was found, and the label says it is not for human use.
No established human dose Details ↓
Recommended Dosing
No established human dose
No human dose exists for this vaccine. It is approved only for animals, its US label says it is not for human use and warns that accidental self-injection could affect fertility and pregnancy, and every study read here was done in pigs, cattle, cats, giraffes, other zoo animals, horses or chickens, with amounts chosen for those animals.
What is it?
Male pigs that are not castrated can store two compounds in their fat, androstenone and skatole, that give the meat an unpleasant smell called boar taint. The usual answer has been surgical castration of piglets. This vaccine offers another route. Gonadotropin-releasing hormone (GnRH, called GnRF on the label) is a small peptide from the brain that tells the pituitary to release the hormones that drive the testes and ovaries. To make the immune system respond to it, the product links a synthetic GnRF analogue to diphtheria toxoid, an inactivated bacterial protein.
After the second injection, antibody levels rise quickly and the pig's own GnRH is mopped up. In the founding 2001 trial in 300 pigs, every vaccinated boar developed antibodies, testes shrank by about half, testosterone fell in more than nine in ten, and boar taint compounds dropped to low or undetectable levels in all of them. Later studies showed the same hormonal shutdown, damaged sperm production and an absence of sexual maturity that lasted at least 22 weeks after the second dose, longer than the label implies; in a side-by-side trial, though, testosterone began to return about eight weeks after the second dose.
Farmers are also interested because vaccinated males eat more and grow faster after the second dose. A meta-analysis of 78 studies found vaccinated males gained weight faster and used feed more efficiently than surgically castrated pigs, with the same low risk of boar taint. The US label also covers gilts kept for slaughter, where it suppresses heat; a meta-analysis of 22 studies found faster growth and heavier carcasses with somewhat more fat. In pigs it did not change the activity of two major liver enzyme families that metabolise drugs.
Veterinarians and zoos have taken the pig vaccine off label to other species. It suppressed testosterone and sperm quality in male cats and heat in most female cats, stopped cycling in dairy cows for about four months, and is used for contraception in giraffes and dozens of other zoo species, where reversibility after long use is not guaranteed and unwanted pregnancies still occurred in about one in eight animals. Horseracing authorities treat it as a doping risk, and blood and urine tests have been developed to detect its use in horses.
The label is blunt about people. It says the product is not for human use and warns that accidental self-injection could affect the reproductive physiology of men and women and may harm pregnancy and fertility, so pregnant women should not give it. There is no human version, no human study and no human dose. Human medicines that act on the GnRH system, such as leuprolide, are different drugs with their own labels.
Mechanism of Action
The product is an immunological castration: it works through the animal's antibodies, not as a drug acting on a receptor. Its effect starts after the second injection and fades as antibody levels fall.
Antibodies against GnRH
All vaccinated boars developed anti-GnRH antibodies, which neutralise the animal's own GnRH (2001 trial); titres peaked about two weeks after the second injection (2013 study).
Sex hormones fall
Testosterone and oestrone sulphate fell, and androstenone and skatole in fat dropped to the levels of surgical castrates (2008 study).
Testes and sperm
Leydig cells were disrupted and sperm production impaired, from mild loss to Sertoli-cell-only tubules, for at least 22 weeks (2009 study).
Temporary
Testosterone began to recover about eight weeks after the second dose in one comparison (2013 study).
Published Research
These studies were read from their PubMed abstracts on October 29, 2026. They are trials and analyses in pigs, two meta-analyses, and off-label studies in cows, cats, giraffes, zoo animals, horses and chickens. None was done in people. Any amounts in the cards are veterinary amounts for the species named, not human doses. The two meta-analyses are titled for 'immunization against GnRF' and carded because their abstracts name Improvac.
Founding trial: 300 pigs, boar taint eliminated
Boars were vaccinated 8 and 4 weeks before slaughter at 23 or 26 weeks of age. All developed antibodies, testes and accessory glands shrank by about half, and androstenone and skatole fell to low or undetectable levels in every vaccinated boar, against significant levels in about half of placebo boars. Vaccinated boars grew faster after the second dose; no injection-site reactions were seen at slaughter.
With porcine somatotropin in boars and gilts
In 224 pigs, vaccination shrank testes and ovaries, raised feed intake and daily gain, and added both lean and fat; combined with porcine somatotropin it raised lean growth more than either alone.
Hormones and boar taint in Swedish pigs
In 120 pigs, vaccination cut testosterone, oestrone sulphate, androstenone, skatole and indole; every vaccinated and surgically castrated pig was below the skatole taint threshold, against eight entire males above it. Vaccinated pigs ate and grew more after the second dose.
How long the effect lasts
Twelve vaccinated pigs kept for 16 or 22 weeks after the second dose had lower testosterone, androstenone and skatole throughout, smaller testes, less aggressive and social behaviour, and no sexual activity, an effect longer than the directions for use imply.
Sexual maturity, testes and sperm
None of the vaccinated pigs reached sexual maturity at 4, 16 or 22 weeks after the second dose, while all controls did by 16 weeks. Leydig cells and sperm production were disrupted throughout, ranging from loss of germ-cell layers to tubules with no germ cells left.
Earlier vaccination in 192 pigs
Vaccinating at 10 and 14 weeks of age controlled boar taint and hormones as well as the standard 16 and 20 weeks, with even smaller reproductive organs at slaughter. IGF-1 sat between castrates and entire males.
Early vaccination: testis structure
Early vaccination caused severe arrest of sperm production, with no sperm in the epididymis in 5 of 8 pigs; with standard timing, only 5% of sperm were normal against more than 70% in controls.
Improvac against Bopriva in boars
Both GnRF vaccines raised antibodies that peaked two weeks after the second dose and drove testosterone below detection for at least six weeks. Around eight weeks, testosterone started to rise with Improvac but not with Bopriva, and Improvac testes looked partly recovered at ten weeks.
No effect on liver drug-metabolising enzymes
In pigs, neither vaccination nor surgical castration changed the activity or expression of the liver enzymes CYP3A and CYP2C, which metabolise many veterinary drugs.
Against surgical castration in 446 pigs
Vaccinated pigs converted feed more efficiently than surgical castrates, tended to have lighter carcasses with less backfat, and did not differ in drip loss, shear force or cooking loss.
US growth trial with Improvest
In 192 pigs from 16 weeks of age, vaccinated barrows grew fastest (1,150 g a day) and reached slaughter weight soonest, ate more than intact males or gilts, and converted feed better than surgical castrates; after the second dose they grew about 31% faster than castrates.
Timing the doses in 1,632 gilts
Gilts given Improvest ate more from about two weeks after the second dose and grew faster for a few weeks; overall they were heavier at slaughter with more backfat and the same loin muscle depth as untreated gilts.
Meta-analysis: 78 studies in male pigs
Against surgical castrates, vaccinated males gained about 33 g a day more with better feed conversion and more lean, but lower carcass weight and dressing percentage. The risk of exceeding taint thresholds was the same as for castrates and lower than for entire males.
Meta-analysis: 22 studies in gilts
Vaccinated gilts gained about 45 g a day more, ate more, had heavier carcasses and about 2.8 mm more backfat, were slightly less lean, and showed no difference in meat colour, pH or drip loss.
Stopping heat in dairy cows
Twenty-one cows vaccinated twice at least 35 days apart had their cycles suppressed for an average of 114 days (range 59 to 175); a booster extended it by about 127 days in three cows. Apart from slight swelling at the injection site, no side effects were seen.
Male cats
In nine vaccinated tomcats, testosterone fell to basal levels by six weeks, and by 24 weeks sperm count was down 90%, motility 70%, testicular volume 49%, and penile spines had almost disappeared. Red-cell values fell but stayed within normal range.
Female cats
Seven of nine vaccinated queens showed no heat until spaying at up to 330 days; two came into heat after vaccination. Painless soft lumps formed at injection sites and later resolved, and ovaries showed follicular cysts with an atrophic uterine lining.
Contraception in giraffes
Females needed three injections and males four or five at four-week intervals to bring hormone metabolites to baseline, with boosters every two to four months after that. Some animals had still not recovered more than two years after stopping.
Survey of zoos in German-speaking countries
Twenty-one zoos reported using the vaccine in 188 animals of 42 mammal species over 11 years, with no consistent schedule. Unwanted pregnancies occurred in 17 animals (12.1%); side effects were local swelling, slight lameness and sexual hyperactivity in some males.
Detecting misuse in racehorses
Four standardbred stallions given two injections had scrotal width fall 37% and anti-GnRH antibodies detectable for up to 200 days. A plasma antibody test and a urine steroid profile were developed to catch its use, which racing rules prohibit.
Male broiler chickens
Vaccinated birds grew faster, had lower testosterone, impaired sperm production and reduced activity of genes for testosterone synthesis, suggesting partial immunocastration as an alternative to caponisation.
No Established Human Dose
No human dose exists for this vaccine. It is approved only for animals, its US label says it is not for human use and warns that accidental self-injection could affect fertility and pregnancy, and every study read here was done in pigs, cattle, cats, giraffes, other zoo animals, horses or chickens, with amounts chosen for those animals. No study giving it to people was found.
What this rests on: PMID 11721830, PMID 29958153, PMID 28294552.
Educational information, not medical advice. The doses on this page come from approved labels, published studies and, where no study exists, amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
What a Buyer Needs to Know
What is sold, how it is mixed, stored and taken, and what is known about its safety, quality and legal status. Each part names its source. Where we have none, it says unknown.
What is sold
No product is approved for people. The approved veterinary product, as its US veterinary label describes it:
- Improvest (gonadotropin releasing factor analog-diphtheria toxoid conjugate solution) (veterinary): IMPROVEST is available in a 250 mL bottle.
Source: Improvest (gonadotropin releasing factor analog-diphtheria toxoid conjugate solution) veterinary label, How Supplied (DailyMed version 7, effective May 5, 2020).
Mixing a vial: a worked example
Unknown: no human dose of GnRF Analogue–Diphtheria Toxoid Conjugate has been established (see No Established Human Dose below), so there is no amount to work out. The reconstitution calculator does the arithmetic for any vial, water and dose.
Storage before and after mixing
No product of GnRF Analogue–Diphtheria Toxoid Conjugate is approved for people. The US veterinary label of the approved animal product says how it is stored:
Improvest (gonadotropin releasing factor analog-diphtheria toxoid conjugate solution) (veterinary): “Store under refrigeration at 2°-8°C (36°-46°F).” “Once broached, product may be stored under refrigeration for 28 days.” “Store bottle in carton until used.” “Protect from light.” “Protect from freezing.”
These describe the veterinary product in its own container, not a vial sold for people. The mixing guide sets out the general rules for a mixed vial, with their sources.
Source: Improvest (gonadotropin releasing factor analog-diphtheria toxoid conjugate solution) veterinary label, storage section (DailyMed version 7, effective May 5, 2020).
How it is taken
Unknown: with no established human dose, there is no established way of taking GnRF Analogue–Diphtheria Toxoid Conjugate. What the human studies gave and how is set out under No Established Human Dose.
What to expect, and when
Unknown: no cited half-life and no label or trial duration on this page says how long GnRF Analogue–Diphtheria Toxoid Conjugate lasts or how long it was given. Results reported in the studies are under Published Research.
Side effects, and when to stop or get help
From the Side Effects section: A transient injection-site reaction that may cause trim loss at slaughter (US label); Soft, painless injection-site lumps in cats that later resolved (PMID 42405487); Local swelling, slight lameness and sexual hyperactivity in some zoo animals (PMID 42684840); Effects that may not reverse after long-term use in giraffes (PMID 34478196); Unwanted pregnancies despite vaccination in about 12% of zoo animals (PMID 42684840). Full list and cautions ↓
When to stop or get help: unknown; no label or trial on file sets rules for stopping GnRF Analogue–Diphtheria Toxoid Conjugate. Treat a severe or unexpected reaction as a reason to stop and see a doctor.
Source: the Side Effects section below (“Reported in animals; no human safety data exist”).
Interactions
Unknown: a PubMed search for human interaction studies of GnRF Analogue–Diphtheria Toxoid Conjugate (reviewed 2026-10-28) found none. Unknown is not the same as safe to combine.
Every peptide's record, with its source: the interaction matrix.
Checking what you bought
Unknown: no published independent test of GnRF Analogue–Diphtheria Toxoid Conjugate products is in our lab-results database yet, so how products sold under this name measure up is not known.
To read a certificate of analysis, paste it into the COA decoder. Trust & Verification explains what independent testing can and cannot show.
Legal status
US: Veterinary Only (Approved only for animals). Sport: Prohibited (S0 catch-all).
Country by country ↓ · Regulatory tracker
Source: this entry's approval status and the WADA Prohibited List, as in Legal Status below.
Side Effects & Contraindications
Reported Side Effects
Reported in animals; no human safety data exist.
Contraindications & Cautions
For people, the main point is that this is not a human medicine.
Legal Status
This product is approved only as a veterinary medicine.
Frequently Asked Questions
?What is Improvac or Improvest?
A veterinary vaccine that joins a synthetic GnRH analogue to diphtheria toxoid so that pigs make antibodies against their own GnRH, which temporarily shuts down the testes or ovaries.
?Why is it used?
To replace surgical castration of male pigs by preventing boar taint, and in the US to suppress heat in gilts kept for slaughter.
?Is it a human medicine?
No. The label says it is not for human use and warns that accidental self-injection could affect fertility and pregnancy. No study has given it to people.
?How long does it work?
In male pigs the effect lasted at least 22 weeks after the second dose in one study, while in another testosterone began to recover around eight weeks.
?Is it used in other animals?
Off label, in cats, dairy cows, giraffes and many zoo species for contraception; it is prohibited in racehorses.
References
- [fda-pi] Improvest (gonadotropin releasing factor analog-diphtheria toxoid conjugate), US FDA-approved veterinary label (description, indications and dosage; DailyMed set 4262802d, version 7, effective May 5, 2020; read October 28, 2026).
- [other] US FDA Center for Veterinary Medicine, Approved Animal Drug Products (Green Book), Section 2 active-ingredient list downloaded from Animal Drugs @ FDA, read October 28, 2026: NADA 141-322, Improvest, active ingredient gonadotropin releasing factor-diphtheria toxoid conjugate. https://animaldrugsatfda.fda.gov/adafda/views/#/search
- [other] European Medicines Agency, medicines list (JSON report, downloaded October 28, 2026): Improvac (synthetic peptide analogue of GnRF conjugated to diphtheria toxoid; gonadotropin releasing factor (GnRF) analogue-protein conjugate): category veterinary, authorised.
- [pubmed] Dunshea FR, et al. "Vaccination of boars with a GnRH vaccine (Improvac) eliminates boar taint and increases growth performance." J Anim Sci, 2001;79(10):2524-35. PMID: 11721830.
- [pubmed] Oliver WT, et al. "A gonadotropin-releasing factor vaccine (Improvac) and porcine somatotropin have synergistic and additive effects on growth performance in group-housed boars and gilts." J Anim Sci, 2003;81(8):1959-66. PMID: 12926778.
- [pubmed] Zamaratskaia G, et al. "Effect of a gonadotropin-releasing hormone vaccine (Improvac) on steroid hormones, boar taint compounds and performance in entire male pigs." Reprod Domest Anim, 2008;43(3):351-359. PMID: 18086253.
- [pubmed] Zamaratskaia G, et al. "Long-term effect of vaccination against gonadotropin-releasing hormone, using Improvac, on hormonal profile and behaviour of male pigs." Anim Reprod Sci, 2008;108(1-2):37-48. PMID: 17714891.
- [pubmed] Einarsson S, et al. "Short- and long-term effects of immunization against gonadotropin-releasing hormone, using Improvac, on sexual maturity, reproductive organs and sperm morphology in male pigs." Theriogenology, 2009;71(2):302-10. PMID: 18762333.
- [pubmed] Brunius C, et al. "Early immunocastration of male pigs with Improvac(®) - effect on boar taint, hormones and reproductive organs." Vaccine, 2011;29(51):9514-20. PMID: 22008824.
- [pubmed] Einarsson S, et al. "Effects of early vaccination with Improvac(®) on the development and function of reproductive organs of male pigs." Anim Reprod Sci, 2011;127(1-2):50-5. PMID: 21802872.
- [pubmed] Wicks N, et al. "Effects of Improvac and Bopriva on the testicular function of boars ten weeks after immunization." Anim Reprod Sci, 2013;142(3-4):149-59. PMID: 24139761.
- [pubmed] Tomankova J, et al. "Improvac does not modify the expression and activities of the major drug metabolizing enzymes cytochrome P450 3A and 2C in pigs." Vaccine, 2012;30(24):3515-8. PMID: 22484351.
- [pubmed] Albrecht A, et al. "Growth performance and carcass characteristics of Improvac-treated male pigs compared with barrows." Berl Munch Tierarztl Wochenschr, 2012;125(11-12):456-62. PMID: 23227762.
- [pubmed] Puls CL, et al. "Growth performance of immunologically castrated (with Improvest) barrows (with or without ractopamine) compared to gilt, physically castrated barrow, and intact male pigs." J Anim Sci, 2014;92(5):2289-95. PMID: 24671576.
- [pubmed] Vasquez-Hidalgo MA, et al. "The effect of timing of Improvest administration on growth performance and carcass characteristics in gilts." Transl Anim Sci, 2023;7(1):txad051. PMID: 37786423.
- [review] Poulsen Nautrup B, et al. "The effect of immunization against gonadotropin-releasing factor on growth performance, carcass characteristics and boar taint relevant to pig producers and the pork packing industry: A meta-analysis." Res Vet Sci, 2018;119:182-195. PMID: 29958153.
- [review] Poulsen Nautrup B, et al. "The effect of immunization against gonadotropin-releasing factor in market gilts: Meta-analyses of parameters relevant for pig producers, pork packers and retailers/consumers." Res Vet Sci, 2020;131:159-172. PMID: 32387811.
- [pubmed] Schmid R, et al. "[Oestrus suppression in a dairy herd by means of anti-GnRH vaccination Improvac®: A prospective field study]." Schweiz Arch Tierheilkd, 2020;162(2):93-100. PMID: 32004139.
- [pubmed] Ochoa JS, et al. "Immunocontraception of male domestic cats using GnRH vaccine Improvac." Theriogenology, 2023;198:211-216. PMID: 36610370.
- [pubmed] Carrasco-Sangache WF, et al. "Effects of an Anti-GnRH Vaccine Improvac on Estrous Activity, and Ovarian and Uterine Morphology in Adult Queens (Felis catus)." Reprod Domest Anim, 2026;61(7):e70268. PMID: 42405487.
- [pubmed] Schwarzenberger F, et al. "Immunocontraception of male and female giraffes using the GnRH vaccine Improvac®." Zoo Biol, 2022;41(1):50-64. PMID: 34478196.
- [pubmed] Hantschmann A, et al. "Contraception of zoo animals using Improvac® in German-speaking countries - A questionnaire survey." Acta Vet Hung, 2026;74(3):284-292. PMID: 42684840.
- [pubmed] Bailly-Chouriberry L, et al. "Two complementary methods to control gonadotropin-releasing hormone vaccination (Improvac®) misuse in horseracing: Enzyme-linked immunosorbent assay test in plasma and steroidomics in urine." Drug Test Anal, 2017;9(9):1432-1440. PMID: 28294552.
- [pubmed] Wang C, et al. "Improvac induces immunocastration by affecting testosterone levels and disrupting spermatogenesis in male broiler chickens." Poult Sci, 2019;98(11):6034-6045. PMID: 31041439.
Sources & Citations
Every study on this page was read from its PubMed record on October 29, 2026. Label details come from the DailyMed veterinary label for Improvest (version 7, effective May 5, 2020) and approval status from the FDA Green Book and the EMA medicines list, read October 28, 2026. PubMed searches for Improvac or Improvest with human, self-injection, accidental or occupational terms found no study in people, and ClinicalTrials.gov lists none. Grey Peptides sells nothing and links to no vendor.
Medical Disclaimer: This content is for educational and informational purposes only. It is not medical advice, and nothing here is an instruction to obtain or use any compound. Talk to a licensed clinician about your own situation.