yahoo answers...looks informative
Hi there...it is very dangerous to give pet animals any human medications unless prescribed by a vet as it could be very lethal. Those that can be taken need a vet to prescribe the correct dosage based on weight, age and various other factors depending on the diagnosis/treatment.
Another reason is that historically, veterinarians shied away from pain killers for cats because this species is quite sensitive to some of the products we humans (or dogs) can routinely look to for pain control. Now, extensive research has proved that some of these products can be used cautiously for cats too, either for short periods, at lengthened dosage intervals, or at a lower dosage levels.
Some pain medicines will never be safe or effective because cats are quite peculiar. Their liver lacks a fully functioning enzyme pathway, one that both canines and humans have. It is a low efficiency of thisglucuronidation pathway that leads to their relative intolerance of the non-steroidal anti-inflammatory (NSAID) class of pain killer medications. Acetaminophen, carprofen, and aspirin are some examples of specific drugs with increased toxicity in cats due to this species-specific deficiency.
Please read the following article written by a University of Illinois Urbana Veterinary about the dangers of human medicine to cats:
 cats have a very high tolerance for pain and generally can suppress the pain to a certain extent. They are very good at hiding the pain and many who are spayed show little signs of distress. You can discuss this with your vet and learn more about this.
Here's an article about signs in cats who are in pain: