While you're waiting for Sqad:
Written by a vet:
"Human drugs are not always safe for pets, so never give anything to your pet without talking to your vet first: half a paracetamol can kill a cat, a big dose of ibuprofen can do the same to dogs and even a solitary aspirin can be a lethal overdose for a toy breed, designed as it is for a much bigger animal (us). This is why we have veterinary-licensed drugs for our patients."
http://www.vethelpdirect.com/vetblog/2011/03/24/pain-part-2-getting-rid-of-pain/
Additionally, while some human drugs may be safe for use with animals, the form that we use them may well be unsuitable. For example, some NSAIDS have unpredictable absorption rates when given in tablet form. That's not usually a problem for humans but could harm other animals. So drugs that we take in tablets might need to be given in liquid form, or injected, with other animals.