Surgery remains the standard way to prevent litters, but it takes time, money, and trained veterinary teams. For that reason, researchers have long hoped a simple shot could do the job instead. One candidate is GonaCon, a birth control vaccine. Two studies from the same team in Israel recently tested GonaCon in cats along with a rabies vaccine. The results show real progress. Yet they also confirm that the vaccine does not prevent reproduction in every cat, and its effect often fades within months. For background on new cat contraceptive methods under development, see our cat health topics.
How GonaCon works
GonaCon trains the immune system to make antibodies against GnRH (gonadotropin-releasing hormone). The brain releases GnRH to tell the body to make sex hormones, sperm, and eggs. GnRH acts as the master switch for reproduction. When antibody levels run high, they block its signal, and sex hormone production drops. However, each cat’s immune system responds differently, so results vary.
Earlier research on GonaCon in cats: mixed results
GonaCon was developed by the USDA’s National Wildlife Research Center as a method to control wildlife populations. GnRH immunocontraceptives have also been tested in many other animals. Two decades of studies in cats show a pattern of partial success. Here is a rundown of previous studies in cats:
- In a 2004 study, six of nine male lab cats responded strongly, and their testosterone dropped to undetectable levels regardless of the dose.
- A 2011 study reported long-term contraception in female lab cats. Of 15 vaccinated cats, 93% stayed infertile for one year. By year four, however, that share had fallen to 40%. Granulomas (lumps of immune cells) occurred at the injection site in five cats after two years.
- Using a newer GonaCon formula, a 2017 safety trial found that six spayed female cats developed an immune response to one or two vaccinations over 6 months. Four cats developed injection site granulomas that later went away.
- In a 2018 colony study, researchers vaccinated 20 females and later introduced fertile males. Within four months, 60% of vaccinated cats became pregnant. By one year, only 30% had avoided pregnancy for a full year. Almost half showed injection-site reactions.
The studies reported different outcomes. Some found strong immune responses and extended contraception and others found only partial short-term contraception. This may have been due to different GonaCon formulations, individual variability, or environmental factors (free-roaming vs. lab housed cats).
The latest GonaCon studies in cats
Why add a rabies vaccine?
In the United States, cats are now the companion animal most often reported with rabies. Many trap-neuter-return (TNR) programs already vaccinate cats against it. Giving both shots in one visit could save time and reduce stress. A 2009 study showed that GonaCon did not weaken rabies protection in dogs. The two new studies asked whether the same holds true for cats.
Study 1: GonaCon in female feral cats
In a 2021 study, researchers at the Hebrew University of Jerusalem selected 16 feral females from a TNR program in Be’er-Sheva, Israel. Every cat was housed in the laboratory for the study. Each received a rabies vaccine. Nine also received GonaCon, either once or twice. The team spayed four of the others and left three intact for comparison. Veterinarians then examined the cats every two weeks for 14 weeks.
No cat showed side effects or injection site issues during the study, and all kept protective rabies antibody levels. Eight of nine vaccinated cats made antibodies against GnRH. In those cats, hormone tests, vaginal swabs, and ovary tissue all pointed to paused reproduction. However, one cat never responded and went into heat. A second dose also offered no clear benefit over one.
Study 2: GonaCon in male pet cats
The 2026 study by the same research team followed 38 vaccinated male pet cats living at home for one year. Four neutered cats served as a comparison group. Researchers tracked testosterone and penile spines, tiny barbs that shrink when testosterone falls. Only 22 vaccinated cats finished the study. Most dropouts were outdoor cats that disappeared, evaded recapture, or died after cars struck them. Four cats developed firm granulomas at the injection site that mostly shrank over time.
Once again, rabies protection held in every cat. The contraceptive effect, however, faded fast. At three months, 17 of 22 vaccinated cats had antibodies against GnRH. By one year, only 3 still did, and 5 cats never responded at all. Where antibodies stayed high, testosterone dropped, and penile spines shrank. But by 12 months, average testosterone had returned to starting levels or higher. Nearly as many cats had penile spines as at the start.
Where the cats lived also mattered. Outdoor cats made weaker, shorter-lived responses to the GnRH vaccine. In fact, four of the five non-responders lived outdoors. Owner opinions followed the same pattern. Every indoor-cat owner was satisfied, compared with only about one in three outdoor-cat owners. The authors concluded that GonaCon may need a stronger formula or added approaches to deliver consistent, lasting results.
What these studies tell us about GonaCon for cats
The good news is clear. In both studies, GonaCon did not weaken rabies protection. That finding supports future programs that combine the two vaccines.
The central problem is also clear: the contraceptive effect is not reliable. Across both studies, about one in five vaccinated cats never responded (one of nine females and five of 22 males). Among males that did respond, most lost protection within six months. And outdoor cats, the ones programs most want to reach, responded least. That matters, because experts estimate that programs must sterilize more than 75% of free-roaming females to stop population growth.
These studies also have limits. Both were small and neither study ran a breeding trial, so we don’t know how many cats could still produce kittens. Instead, researchers relied on hormones and physical signs, which offer only indirect clues.
The takeaway on cat contraception
Research on GonaCon in cats continues to advance. These studies show that programs can safely pair it with rabies vaccination. They also show it can pause reproduction in some cats for a few months. But it does not work in every cat, and the effect rarely lasts a full year. Larger studies and more effective formulations would be needed to advance GonaCon for cat contraception.
As our last review of nonsurgical pet contraception concluded, no current option can yet replace surgery. However, scientists are working to develop better options. The recent advancement of gene therapy to stop ovulation in cats by increasing the production of anti-Müllerian hormone shows real promise.
To stay up to date on non-surgical contraceptives, visit the Alliance for Contraception in Cats and Dogs.
