Called Neutersol, it's a shot administered directly into the testicles of puppies at the right age for neutering. The ingredients - the amino acid l-arginine and a zinc salt - cause the testicles and prostate to atrophy.
The alternative, surgically removing the testicles, guarantees sterility. In a study of 224 dogs, Neutersol did almost as well: Semen analysis showed only one sterilization failure, said Dr. Melanie Burson of the Food and Drug Administration.
The FDA approved Neutersol in March, but did not announce the approval until Monday.
A type of chemical castration already is available for humans. Sex offenders often are sentenced to regular injections of hormones that deplete testosterone. But that's not permanent sterilization.
The new method for dogs is permanent, but some testosterone production continues because it's not hormonally driven, Burson said. So unlike surgical castration, Neutersol may not eliminate unwanted male behaviors such as roaming, marking and aggression, the FDA warned.
Pet overpopulation is a serious problem, and scientists have struggled to find ways to make animals infertile. Some were dangerous either for the animal or for the veterinarian administering them, said Bruce Addison, founder of Addison Biological Laboratory of Columbia, Mo., which is selling Neutersol.
For some reason, Neutersol ingredients are toxic to cells when injected directly into the testicles but not other parts of the body, Addison said.
It is crucial for vets to administer the shot properly, and for owners to care for the puppy during the following week to avoid ulceration and infection of the injection site, the FDA said.
Scientists now are studying whether Neutersol can safely sterilize older dogs and cats, Addison said.