Acetaminophen is a non-aspirin pain reliever found in more than 200 formulations, but the most recognized trade name is Tylenol. Acetaminophen is often found in combination with other drugs, such as antihistamines and decongestants. Cats are most sensitive to the effects of acetaminophen, although dogs can also be poisoned. Cats lack an enzyme necessary to detoxify acetaminophen, so it can accumulate quickly in the bloodstream. Because of this, the ability of the blood to carry oxygen is most commonly affected in cats. The blood turns a brown or gray color and is unable to transport oxygen to body tissues. This condition is called methemoglobinemia. Dogs generally require higher doses of acetaminophen to be poisoned, and the liver tends to be the most affected organ.

Within two to four hours after exposure, cats may develop a purplish-brown or gray discoloration to the gums, accompanied by weakness, rapid heart rate and rapid breathing. In some cases, cats drool, vomit or develop swelling of the face and paws. Blood may appear brown, gray, or black, and the urine may be discolored. Clinical signs of anemia and decreased urine production from kidney failure are also possible. In dogs, vomiting and jaundice (which is a yellow discoloration to the eyes) may occur with liver toxicity. Dogs may also develop a reduction in the ability of the blood to carry oxygen but at higher doses than those that cause liver failure.

A tentative diagnosis is based on a history of exposure to acetaminophen, consistent clinical signs and characteristic discoloration of the blood or urine. Laboratory tests often indicate an anemia in cats and liver damage in dogs. Specialized blood tests to detect methemoglobinemia or to measure acetaminophen levels can be performed at outside laboratories.