TL;DR. Cats lack the liver enzyme (UDP-glucuronosyltransferase) needed to safely metabolise paracetamol. Even a single human tablet can be fatal. This is just one example of why pet medications must be species-specifically compounded — not approximated from human or canine versions.
The biology
In humans and dogs, paracetamol (acetaminophen) is primarily processed in the liver by conjugation with glucuronic acid, producing a water-soluble metabolite that’s safely excreted. Cats produce only ~10% of this enzyme. The unprocessed paracetamol instead converts to NAPQI (N-acetyl-p-benzoquinone imine), a highly reactive toxin that destroys red blood cells (methaemoglobinaemia) and liver tissue.
A single 500 mg paracetamol tablet — a normal human dose — can kill an average 4 kg cat. Owners who don’t know this often unknowingly poison their cats with “gentle” fever relief.
What veterinary compounding solves
- Species-appropriate doses. A 5 kg cat needs roughly 10% of a 50 kg dog’s dose. Splitting tablets isn’t accurate enough.
- Safer drug substitutions. For pain in cats, buprenorphine, gabapentin, or compounded NSAID alternatives — not paracetamol.
- Flavour-driven compliance. A cat won’t swallow a tablet but will eat tuna-flavoured liquid medication.
- Transdermal options. Methimazole transdermal gel applied to the ear flap works for hyperthyroid cats who won’t take oral therapy.
Other species-specific risks
- Dogs: xylitol (in human sugar-free products) causes catastrophic hypoglycaemia; grape, raisin, chocolate are also toxic
- Rabbits, guinea pigs: oral penicillin causes fatal enterotoxaemia
- Birds: very narrow therapeutic windows; doses calculated to fractional mg
If your vet has prescribed a compounded medicine for your pet, see our veterinary compounding page for what we do and how to send the prescription.
Medically reviewed by Vitthia Rama Murti, BPharm Hons (University of Cyberjaya), RPh 15632 — Last reviewed 27 May 2026.