Friday, April 1, 2011

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!


Rachel said...

i understand your frustration when your techs say "all OTC drugs are toxic"; however, from the tech's POV...clients have a nasty habit of wanting to treat their pets cheaply by using OTC drugs. i find that many clients who call in just simply want to relieve their pet's pain, and they are not familiar with generic NSAID names, mechanisms of action, and the ability or inability of their cat or dog to metabolize a particular drug. the client just knows that they have Aleve in their cabinet, and it relieves their own back pain, so maybe it'll work on their dog who's non-weight bearing on one of their hind legs. it's very frustrating that clients call AFTER the fact and say, "oh by the way, i gave him an Aleve." to combat this, many techs, like myself, simply state that "many OTC drugs can be quite toxic and are not recommended." and i also like to follow this up by saying "there are veterinary prescriptions [Rimadyl/Deramaxx] that your vet may want to use, and these can't be given for several days if you give your pet aspirin or other NSAIDs." as a tech, i know that there is more to the story than simply saying, "all OTC drugs are toxic" but i also have to think about what the client is going to remember when they get off the phone, and who they're going to talk to and communicate this info to. in the end, i think it's better if clients know to simply stay away from OTC NSAIDs, and hopefully this is what they pass on to their family and friends.

by the way, i LOVE reading your blog. i work at an emergency/specialty/referral hospital, and while i generally don't enjoy watching or reading work-related TV or postings on my days off, there is something about your blog that i am drawn to. i can feel from your writing how much you care about your patients.

The Nurse said...

Just a clarification please.....dogs can have aspirin but not buffered aspirin? And Tylenol-3 is tylenol with codeine....this is okay to give to dogs? Of course, although I'm curious, I'm the one who never administers medications without a vet's approval!

The Homeless Parrot said...

Rachel: you're right about the fact that it's better to just give people the worst case scenario. On the other hand, I just hate to give out misinformation. It makes us look bad.

Nurse: I meant that normal aspirin is not buffered for dogs, buffered aspirin is better for dogs. Tylenol-3 is used for dogs, as well - but usually, it is reserved for oncology patients and the like.

booksdogsandfrogs said...

Blogger hates me today. I wrote a long comment that it ate, on how I dislike it generally when the vet tells me to give my dogs OTC drugs, because it is harder to get the right dose and the vet never knows what specific product I should get. Once he recommended giving just plain cough syrup, but I couldn't find a brand that was just pain cough syrup so my dog did without. It was frustrating because I would have gladly spend 20-30$ to buy drugs from the vet. I think my vet just does this with things where the dosage is less sensitive then the otc pain meds you talk about, but I'm not a vet and figuring out how to get 250 mg out of a 1500mg pill for a daily dose is more trouble then it is worth.

The Homeless Parrot said...

Books: Have you told your veterinarian that you would rather pay for the veterinary product and correct dosing? I think many DVMs do this because in most instances, owners would rather not pay for the medicine and just give something out of the medicine cabinet. I think if you made it clear to your vet - she/he would be happy to oblige!