Book excerpt: Increasingly, veterinarians are called on to treat behavioural disorders in companion pets such as cats, dogs and horses, which can often develop aggressive compulsive or other antisocial habits. In America, veterinarians are beginning to treat these disorders with psychotropic drugs such as Prozac, which have already been shown to yield good results in humans. In the UK, these drugs cannot yet be used on animals, but a number of pharmaceutical companies are developing derivatives for use in animal practice and these products are eagerly awaited. The effects are expected to be as far-reaching as tranquillisers and antidegressants were when they were introduced into human medicine.