I found a website form the UK about this... it says,
Male dogs can be neutered, castrated, when they are 6 months or older. The castration procedure is carried out under general anaesthetic. The operation involves the removal of the testicles. There can be several reasons why a male dog has to be castrated. This can be for a medical reason or to stop excessive sexual behaviour and other unacceptable behaviour characteristics.
Medically the dog may only have one testicle descended into the scrotum, the other could be in the abdomen. If this is the case the dog should be castrated as the testicle in the abdomen could become a tumour and upset the whole hormonal system. A tumour of this type can be life threatening if it spreads to other parts of the body. Behaviourally a dog can, if highly sexed mount people and inanimate objects around the house. Unneutered males can be aggressive, in particular with other dogs, they mark their territory, in the house and outside. They will also wander off, especially if they sense there is a bitch in season nearby. Sometimes castration will be ineffective as the behaviour has become a habit. Other behaviours can take several months before any change is noticed.
There are some disadvantages in having your dog neutered. A castrated dog can have a change in the appearance and texture of his coat. Some of the hairier breeds can have excess hair over the hindquarters and the smooth coated breeds can develop bald patches. The other problem that can occur is weight gain, with some breeds being more susceptible than others. Once neutered try and cut out the treats, and increase the amount of exercise.
The control of the dogs sexual behaviour can also be achieved without surgery.
There is available a hormone injection which is administered by your vet. This injection mimics the effects of castration, however the dog may not become infertile. There are various commercial products, like sprays and lotions, which can be used on the areas where the dog is scent marking around the house. These products have various degrees of success.
That would prove the injection exists
. It also says this will be beneficial if you want to show your dog. However, the one testicle problem will require a neuter eventually. Hope this helps!