Yes there are two different ones - one is called Tardak, which is short lived (only lasts a few weeks) and is often used to see if castration would work for things such as aggression. The other is called Suprelorin, which is a slow release implant which lasts either six months or a year depending which one you give. During the time is it in use the dog loses all instincts to go 'womanising' and it renders them infertile, but once the six or twelve months or so has passed the dog reverts back to normal. His round bits also shrink while on it, but they revert back to full size after. It is very useful if it is planned to breed from the dog later in life, and I have used it on several dogs over the years. What it does not do is correct any behaviour issues such as aggression or extreme hyper behaviour, it is really intended just to prevent them showing a lot of interest in the girls and fathering puppies. The vet would happily explain. It costs around £80 for the six month jab. If the dog is never going to be bred from I would personally wait till he was mature (at least eighteen months) and then have him castrated.