What's Good for the Goose Isn't Necessarity Good for the Gander
When veterinarians prescribe a medication for a pet, they do so not only knowing the needs of the specific animal it is for, but also knowing what is safe and what isn’t. Medications should only be given to the pet for whom it was prescribed.
Sometimes people will hang on to medication, and think “Oh well, this was ok for Scruffy, so it must be ok for Fluffy.” This can have serious, and sometimes deadly, effects.
There are medications that are safe for dogs, but not for cats. And dogs almost always are prescribed a higher dosage of medication than cats. Some medications - such as acetaminophen and ibuprofen (Tylenol and Advil) - can often be deadly to cats. And if you give Fluffy an antibiotic originally prescribed for your dog, you can have problems ranging from kidney damage to neurological disease, and even death.
The moral of the story is...give your pet only the medication he/she was prescribed. And always check with a veterinarian before giving your pet any other kind of medication.