Vicodin and most other human painkillers should never be administered to pets.
In fact, unless your veterinarian specifies otherwise, never give any sort of human medication to your pet. Also, don’t give canine medicines to cats. And don’t give feline medicines to dogs.
Dogs and cats react to and metabolize medicines in unique ways. Medications that are generally safe for people can be toxic to pets. The acetaminophen (also known as Tylenol or paracetamol) in Vicodin is especially dangerous to cats. In dogs it has a low margin of safety.
Cats and small dogs are especially vulnerable to adverse effects from human medicines. Small pets easily can be over dosed with medicines that might otherwise not cause problems. Severe complications, including death, are possible.
The take-home message is very simple. Do not ever give any medication to your pet without first consulting your vet.