Cats and Tylenol

It annoys me no end when I hear my technicians on the phone telling people that all over-the- counter medications are deadly to pets. They do this because we are not allowed to dispense medication advice over the phone. If someone calls wanting a dose of medication for a limping pet, we do not give that information. If we are busy, and I field the call (as sometimes happens), I simply say, "without evaluating your pet, it is not possible to determine the cause of the problem. Many OTC medications are not safe or buffered for pets, so we recommend that you have your pet seen by a veterinarian so that an appropriate medication can be chosen."

Drugs that can be used in dogs include aspirin (though not buffered for the GI tract) and acetaminophen (used in Tylenol-3 for oncology patients). Drugs that are not safe include ibuprofen and naproxen. Naproxen in particular is a nasty, nasty drug with a very low level needed for toxic events. I once lost a patient to that particular drug, and it was terrible.

Cats, on the other hand, are very intolerant of most OTC medications. Aspirin, ibuprofen, and naproxen are not safe for them, although not always fatal. Acetaminophen (Tylenol) is an exception to this. It is fatal to cats and should never, under any circumstances, be administered to them!

Tylenol has always been an interesting drug to me. Unlike the NSAIDs (aspirin, ibuprofen, naproxen, etc), which work by inhibiting inflammatory pathways, Tylenol's mechanism of action is not completely known. Some argue that it works on the same pathways as NSAIDs (and thus, classify it with aspirin and the like), and some argue that it does not (and classify it as not an NSAID). It was discovered sometime in the last 1800s but did not find wide marketing until the 1950s. It is associated with liver failure in humans, especially when combined with alcohol (sometimes even small amounts).

The body (human and cat) metabolizes Tylenol in the liver. This is primarily through glucuronidation. Glucuronic acid is a chain of carbohydrates that link to the molecule and render it more water soluble. Thus, it can be excreted in urine or feces. There are other pathways through which it is metabolized, but this is the most significant. The problem for cats is that they lack an important enzyme called glucuronyl transferase, which catalyzes this reaction. Without it, the Tylenol does not get metabolized. Without going into too much biochemical detail, the Tylenol is eventually partially metabolized by other pathways, producing some nasty byproducts. These byproducts damage red blood cells, making it difficult for them to release oxygen to tissues. This is called methemoglobinemia.

The hallmark of this is chocolate brown mucus membranes. Roll up a cat's lip with this toxicity, and you will see dark brown gums. The cat will often have trouble breathing due to the red blood cells holding tightly to their oxygen and not releasing it. They sometimes vomit and have a swollen face as well.Further, as in people, Tylenol damages the liver of the cat (although less so than in dogs that have received an overdose).

Therapy is aimed at helping the body metabolize the Tylenol. This is achieved with a drug called Mucomyst (N-acetylcysteine). Other therapy is mainly supportive: oxygen, sometimes blood transfusions, Vitamin C, and other treatments. The prognosis can be good to very grave - depending on how long it has been since ingestion of the toxin (and how much).

Personally, I have yet to treat one of these, but almost every older ER doctor I know has, so it's coming. And when it does, I shall be ready!! The moral of this story: no Tylenol in cats! Consult your veterinarian before administering anything OTC. Dogs and cats are not small people. The doses and safe ranges are very different, and no OTC medications should be administered until you have consulted a vet!