Vet at FirstVet answers:
Chemical castration is performed by placing a hormonal chip (Suprelorin) under the dog’s skin. There are 2 different chips available, one that lasts for 6 months and another that lasts for 12 months. The active ingredient is a hormone that down regulates testosterone causing the testicles to reduce in size. The testicles regain their size and produce testosterone and sperm again once the chip beings to lose its effect.
In addition to a swelling at the injection site, the side effects of chemical castration are the same as those seen with a surgical castration. The chip is not recommended for dogs used for breeding, since there is not a 100% guarantee that fertility will return. The pro with chemical castration is that it is reversible. If you think the dog has reacted in an undesirable way you can rest assured that your dog will return to its normal self once the chip’s effect has worn off. The chip can also be used prior to surgical castration to see if castration will work to dampen down an aggressive dog's behaviour. Previously, progesterone injections or tablets were used to chemically castrate dogs. This is no longer customary due to side effects such as diabetes and mammary cancer.
When a dog is surgically castrated it needs to be anaesthetised and the testicles are surgically removed. There are always risks associated with anaesthesia and complications after surgery such as bleeding and infection. The pro is that it is a one off procedure and does not require continuous hormonal treatment.
The side effects seen with both chemical and surgical castration include: increased tendency to gain weight and changes in coat quality (usually fluffier).