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Human Painkillers and Pets: Ibuprofen, Naproxen, Aspirin and Paracetamol

Updated October 21, 2026 · By the PureOrganism editors

Few households are without a packet of ibuprofen, paracetamol (called acetaminophen in the United States) or aspirin. They are safe enough for most adults, so it is easy to assume that a half tablet "can't hurt" a sore dog or that a dropped pill is a trivial matter. Neither is true. Dogs and cats process these drugs differently from people, and cats in particular are unusually sensitive. This guide explains what the Merck Veterinary Manual says about the doses that cause harm, how the drugs hurt the body, and what to do if a pet gets into the cabinet. It is general information from a website editor, not veterinary advice, and it intentionally contains no treatment doses and no advice to give any human medicine to an animal.

First, the rule that prevents most accidents

Do not give your pet a human painkiller unless a vet who has examined the animal has told you to, and has told you the exact product and amount. The doses that help a person are not scaled-down versions of safe pet doses, and the margin between "might help" and "harmful" is narrow for several of these drugs. Vets prescribe pet-specific medicines precisely because they have been studied in those species. The other half of the rule is storage: most poisonings are accidental. They happen when a pill rolls under the sofa, a bag is left open, or a child shares a "treat".

How NSAIDs harm dogs and cats

Ibuprofen, naproxen and aspirin are non-steroidal anti-inflammatory drugs (NSAIDs). They reduce pain and inflammation by blocking enzymes that make prostaglandins, but prostaglandins also protect the stomach lining and keep blood flowing to the kidneys. A 2026 veterinary journal summary explains that blocking them weakens the gastric mucosal barrier and disrupts the regulation of kidney blood flow. So the usual problems are stomach and intestinal irritation or ulcers, and kidney injury. Gastrointestinal signs, such as vomiting, black tarry stool, poor appetite and abdominal pain, are the most frequent.

Published figures for dogs

The Merck Veterinary Manual lists doses at which effects have been reported in dogs. They come from specific studies and cases and should not be treated as safe limits:

Two things stand out. First, repeated low doses can do harm: the stomach ulcer figures for ibuprofen and naproxen are for days of dosing, not a single event. Naproxen shows effects at much lower doses than ibuprofen does: 5.6 mg/kg daily for a week was enough for vomiting and tarry stool in the reported dogs, against 100 mg/kg or more for a single ibuprofen dose. Second, single-dose toxicity depends heavily on tablet strength and the dog's weight. A worked example, with invented numbers: a 5 kg dog swallows four 200 mg ibuprofen tablets, a total of 800 mg. 800 ÷ 5 = 160 mg/kg. That falls between the 100 to 125 mg/kg stomach range and the 175 to 300 mg/kg level where kidney failure may occur, so it is a serious exposure that warrants an immediate call. The same four tablets in a 30 kg dog give 800 ÷ 30 ≈ 27 mg/kg, well below the single-dose figures but above where repeated dosing has caused ulcers, so it still deserves a call. Our ibuprofen toxicity reference lets you try numbers of your own.

Cats are different

The same Merck page gives no exact mg/kg thresholds for ibuprofen, naproxen or aspirin in cats. It says cats are susceptible to ibuprofen at about half the dose that affects dogs and that they may be more susceptible than dogs to naproxen. Because there is no comfortable number to quote, a cat that has swallowed any NSAID tablet should be treated as an emergency call. Do not give a cat aspirin or any other human painkiller at home.

Paracetamol (acetaminophen): the cat problem

Paracetamol works differently and carries a particular risk for cats. Merck explains that cats have limited ability to glucuronidate the drug, so it accumulates. Toxicity generally occurs with ingestions of 40 to 50 mg/kg, and some cats have shown signs at 10 mg/kg. The main problems are methemoglobinemia (a change in the blood that interferes with oxygen carrying, which develops within a few hours) and liver damage, with kidney injury also possible. For a typical 4 kg cat, 10 mg/kg is 40 mg; a standard 500 mg tablet is therefore many times that amount, and even a fragment of a tablet is a concern. That arithmetic is the reason a "bit of Tylenol for the cat" is dangerous. Our feline acetaminophen reference shows how little it takes.

Dogs tolerate paracetamol better. Merck says acute clinical signs are not observed unless the dose exceeds 100 mg/kg, and that methemoglobinemia has been reported above 200 mg/kg; liver damage and jaundice are more common in dogs than in cats. Repeated exposure can cause toxicity at lower doses. Be careful with combination products: many cold, flu and migraine tablets pair paracetamol with other drugs, and the label may not make the quantity obvious.

Other hidden risks in the same cabinet

• Coated and chewable tablets. Flavoured chewables are appealing to dogs, and a blister pack of them is easy to chew through.
• Liquids and gummies. Some liquid medicines and gummy vitamins contain xylitol; see our xylitol guide.
• Patches, gels and creams. Topical NSAIDs can be licked off skin, and used patches are attractive to dogs. Ask a helpline about any product left on the floor or bin.
• Combination products. Cold and flu tablets, sleeping aids and migraine tablets mix active ingredients, so the label matters more than the brand name.
• Medicines for other pets. A medicine prescribed for one animal can harm another in the household, or the same animal at a different dose.

Spotting trouble

NSAID signs include vomiting (sometimes with blood), black tarry stool, reduced appetite, abdominal pain, lethargy, and increased thirst and urination as the kidneys are affected. Paracetamol in cats may bring depression, drooling, vomiting, swelling of the face or paws, and gums that look brown, grey or blue. The Merck page names methemoglobinemia as developing within a few hours. These lists are not a reason to wait: call as soon as an ingestion is suspected, and describe what you know about the product, strength and amount.

What to do

1. Secure the animal and any remaining pills, and count what is missing from the strip or bottle.
2. Read the label for the active ingredient and the strength per tablet, and note the time.
3. Weigh your pet or use a recent weight.
4. Call a vet or poison service at once, and follow the instructions. Do not induce vomiting unless told to. The first-hour guide explains the steps and why.
5. Do not give any human or leftover medicine as "first aid".

Treatment for these exposures is complex, may involve hospital care, and is for a vet to manage, so we do not outline doses or drugs.

Why "just a little" is a poor plan for pain

Owners who reach for the cabinet usually do so because they see a pet in pain: a limping dog after a long walk, an older cat who is stiff. The impulse is kind, but the pattern in the Merck figures argues against it. Repeated modest doses, the kind a well-meaning owner might give over several days, are exactly what produced ulcers in the studies listed for ibuprofen, naproxen and aspirin. A pet that is already dehydrated, elderly or on another medicine may be less able to cope. Pain in animals is also a symptom, and the cause may need a diagnosis; masking it with an unsuitable drug can delay that. If your pet seems to be hurting, a call to your own vet is the quickest safe route, and many clinics will advise on what can be done the same day.

Prevention in practice

Keep all medicines in a closed cabinet, not in a bedside drawer or handbag. Take tablets out of bags the moment you get home; handbags left on the floor are a classic route (see the pet-proofing checklist). Count out a person's weekly pill organiser away from the pet. Pick up any dropped pill immediately. When you have a vet-prescribed medicine for your pet, give exactly what was prescribed and keep it separate from your own. Let your vet know about everything the animal takes, including supplements, so interactions are not missed. Ask visitors to keep their medicines out of reach.

If your pet has swallowed a human medicine

This article is general information, not veterinary advice. Contact a vet or poison service straight away: ASPCA Animal Poison Control +1 888-426-4435, Pet Poison Helpline +1 855-764-7661 (fees may apply), or in the UK the Animal PoisonLine on 01202 509000 (charges may apply). If your pet is vomiting blood, has black stool, breathing difficulty, tremors, collapse or blue or brown gums, go to an emergency vet immediately.

Sources

• Merck Veterinary Manual: Toxicities from human analgesics (NSAIDs and acetaminophen)
• ASPCA Animal Poison Control: What to do if your pet is poisoned
• PDSA: Animal PoisonLine
• Merck Veterinary Manual: Xylitol toxicosis in dogs

Not veterinary advice. The authors are not veterinarians. If your pet may have eaten something toxic, call your vet or a pet poison line now: ASPCA Animal Poison Control (US) +1 888-426-4435, Pet Poison Helpline (US/Canada) +1 855-764-7661, Animal PoisonLine (UK) 01202 509000. Fees may apply. Spotted a mistake? Email huhuwen09@gmail.com with the page address and we will check and correct it.