Acetaminophen (Tylemol)
This one reliably kills cats. 
For some reason, I’ve had some clients give their cats this medication to help bring down fevers. The problem is that this drug causes liver damage and methemoglobinemia (damage to the red blood cells). A single tablet given to a cat can cause death within forty eight hours.
For dogs, it can be used, but overdosing is an issue. A regular strength tablet (325 mg) can seriously harm a 14 pound dog. Intensive treatment and specialized medications are required to counteract the toxic effects of this drug.
Ibuprofen (Advil)
The number one hazard for dogs is the human anti-inflammatory ibuprofen. Widely used in people, this medication has a very narrow safety margin in dogs. A single 200 milligram tablet given to a 17 pound dog can trigger severe gastrointestinal ulceration which causes inappetance, vomiting, and abdominal pain.
A little more shuts down the blood supply to the kidneys, causing acute renal failure and death. At even higher doses, dogs develop nervous signs including depression, seizures and coma.
Dogs find ibuprofen’s sugar coating so attractive that they don’t usually stop after eating one tablet; they consume an entire bottle. This can kill dogs. Ibuprofen should never be given to dogs (or cats).
Acetylsalicylic Acid (ASA or Aspirin)
This is a weak painkiller that works poorly and it can have severe side effects. It can cause depression, vomiting, and anorexia if too much is given. It can also trigger significant gastrointestinal bleeding.
Cats metabolize ASA differently. It can kill them at a very low dose.
Dr. Louise Janes D.V.M. & Dr. Jeff Grognet D.V.M.
Mid-Isle Veterinary Hospital
5-161 Fern Road West
Qualicum Beach, BC
Tel (250) 752-8969
Mid-Isle Veterinary Hospital