Vet at FirstVet answers:
It is OK to give a healthy, adult dog cat food (please observe cat food should not be given to puppies). If a dog is sick it is likely to be inappropriate to feed it cat food and one should consult a veterinarian if that is the case. In general, cat food has a higher fat and protein content than dog food, making it difficult for dogs to digest and also increases the risk of excessive weight gain.