Human medicines (paracetamol/ibuprofen): toxic to animals

Why are human painkillers dangerous for animals?

Paracetamol and ibuprofen are the most commonly used painkillers in humans — but for cats and dogs they are serious poisons. This also applies to small quantities intended as a 'test dose'. The metabolism of animals differs fundamentally from that of humans, causing toxic substances to accumulate rather than being broken down.

Paracetamol

Cats are particularly vulnerable. They lack sufficient glucuronidase capacity — a liver enzyme that humans use to render paracetamol harmless. As a result, a toxic metabolite (NAPQI) accumulates, resulting in:

• Methemoglobinaemia: haemoglobin can no longer transport oxygen, visible as brown or grey mucous membranes
• Heinz body anaemia: red blood cells are damaged
• Liver damage: centrilobular necrosis

Even at low doses this can be fatal in cats [1][4]. In dogs the toxic threshold is higher, but severe liver damage remains a real risk at higher doses. Keratoconjunctivitis sicca (dry eyes) has also been described in dogs [1][2].

Ibuprofen

Ibuprofen inhibits COX enzymes, reducing the production of prostaglandins. Prostaglandins protect, among other things, the gastric mucosa and renal perfusion. In dogs, gastric ulcers can develop even at relatively low daily doses; at higher doses kidney damage follows, and at very high doses CNS depression and convulsions occur [1][3].

Cats are more sensitive to ibuprofen than dogs. A standard 200 mg tablet can already be dangerous for an adult cat weighing 4 kg [3].

Overview per substance and species

There is no safe 'test' dose

Factors such as weight, existing liver or kidney disease and the exact formulation (ordinary tablet vs sustained-release tablet) make it impossible to assess based on packaging whether something is safe for an animal. Never give human painkillers to dogs or cats without a veterinary prescription [1][2].

What to do in case of ingestion?

Contact the veterinary surgeon or an animal poison control centre immediately. The sooner treatment is started, the better the prognosis.

• Paracetamol poisoning: treatment includes N-acetylcysteine to neutralise NAPQI, and agents against methemoglobinaemia. This is time-critical [2][4].
• Ibuprofen poisoning: gastric protection, fluid therapy and support of renal function are the cornerstones [1][3].

Inducing vomiting should only be done on the advice of a veterinary surgeon — in some situations it is dangerous or counterproductive.

'Familiar to humans' does not mean 'safe for animals'

The belief that a familiar household remedy "cannot be that bad" is a common cause of accidental poisoning in pets. The fact that a product is available over the counter for humans says nothing about its safety for other species [1].