Human Medicines That Poison Pets: What Every Owner Must Know

Why is an animal's body different from a human's?

The difference is not merely one of size. Drugs are broken down in the liver by specialized enzymes, and the makeup of these enzymes differs fundamentally between species. What your body clears within hours may remain in your animal's body for days, accumulating until it reaches a toxic level.

• Deficient conjugating enzymes in cats: cats have low glucuronyl transferase activity — the enzyme responsible for binding many drugs to glucuronic acid before excretion. This deficiency alone explains the toxicity of a large number of drugs in cats specifically.
• Differing half-lives: some drugs persist in the dog many times longer than in humans, so repeated doses rapidly accumulate to a toxic level.
• Genetic sensitivity in specific breeds: some herding dog breeds carry a mutation in a drug-transporter gene that protects the brain, so certain drugs penetrate it at concentrations that cause neurotoxicity at doses other dogs tolerate.
• Small body weight: a tablet designed for a 70 kg human represents an enormous dose for a 4 kg cat — roughly seventeen times more if calculated per kilogram.

First: painkillers — the greatest danger

Painkillers are the most common cause of drug poisoning in pets, because they are present in every home and because owners assume they are "mild". They are in fact the most dangerous items in the medicine cabinet.

Paracetamol (acetaminophen) — the cat killer

Paracetamol is normally excreted after conjugation with glucuronic acid or sulfate. Because cats lack sufficient glucuronyl transferase activity, they rely on the sulfation pathway alone; when that pathway saturates, the drug is converted by cytochrome enzymes into a toxic metabolite (NAPQI) that attacks red blood cells and liver cells.

According to the Merck Veterinary Manual, toxicosis in cats generally occurs at ingestions of 40–50 mg/kg, but cases have been reported with clinical signs at only 10 mg/kg. Since a standard human tablet contains 500 mg, a single tablet far exceeds the toxic threshold for an average-weight cat.

Signs of poisoning that should send you to the clinic immediately:

• Mucous membranes (gums and tongue) turning brown or muddy — a characteristic sign of methemoglobinemia.
• Rapid heartbeat and rapid, deep breathing.
• General weakness, marked lethargy, and loss of activity.
• Swelling of the face and paws (common in poisoned cats).
• Later yellowing of the eyes or gums — evidence of liver injury.

Human NSAIDs (ibuprofen, naproxen, diclofenac, aspirin)

These drugs inhibit enzymes that protect the stomach lining and maintain kidney perfusion. In dogs and cats, doses far below what a human tolerates are enough to cause gastric and intestinal ulcers that may perforate, along with acute kidney failure. Cats are more sensitive than dogs because they metabolize this class slowly.

• Naproxen is especially dangerous in dogs because it persists in the body far longer than in humans, so repeated doses accumulate.
• Aspirin is no exception: its use in cats requires very long dosing intervals determined by a veterinarian, and giving it on a human schedule causes toxicosis.
• Warning signs: vomiting (possibly bloody or coffee-coloured), black tarry stools, loss of appetite, abdominal pain, excessive thirst and urination.

Second: other medicines common in every home

Antidepressants, anxiolytics, and sleep aids

Ingesting a human dose of serotonin-reuptake inhibitors or tricyclic antidepressants may cause "serotonin syndrome": tremors, agitation, elevated body temperature, tachycardia, and it may progress to seizures. Some of these drugs are genuinely used in veterinary medicine, but at doses entirely different from human ones.

Decongestants and combination cold medicines

Be especially wary of "combination" medicines that pack a painkiller, an antihistamine, and a decongestant into one tablet — one ingredient may be relatively safe while another is lethal. Stimulant decongestants raise blood pressure, accelerate the heart, and cause severe agitation in dogs and cats.

Medicated skin creams

A hazard everyone overlooks: you apply a medicated cream to your skin, and the dog or cat licks it off. The American Veterinary Medical Association notes that fluorouracil — a chemotherapy drug found in certain medicated skin creams — is a leading cause of poisoning deaths in dogs. The same applies to psoriasis ointments and hormone creams.

Supplements and vitamins

• Iron supplements and prenatal vitamins: an iron overdose causes severe gastrointestinal irritation followed by liver injury.
• High-dose vitamin D: it raises blood calcium and leads to kidney failure and tissue mineralization.
• Sweet-flavoured gummy vitamins: animals swallow them in large quantities because they resemble candy, and many contain xylitol.

Xylitol: not a drug, yet it sits in the medicine box

Xylitol is an artificial sweetener that is highly toxic to dogs. The US Food and Drug Administration warns that it is found in gum, candy, nut butters, toothpaste, mouthwash, and dietary supplements, as well as in prescription and over-the-counter medications — meaning your animal could be poisoned by a seemingly "safe" medicinal syrup.

Symptoms can begin within as little as twenty minutes: vomiting is usually first, followed by signs of low blood sugar — lethargy, weakness, staggering, incoordination, collapse, and seizures. In some dogs the condition may progress to liver failure and fatal bleeding disorders.

Not only human drugs: one animal's medicine given to another

An equally dangerous and common mistake: giving an animal a veterinary drug designed for a different species.

• Topical parasiticides containing permethrin and intended for dogs are strictly contraindicated in cats; cats are far more sensitive to pyrethroids, and applying a dog pipette to a cat causes severe tremors and seizures that may be fatal.
• Veterinary NSAIDs licensed for dogs are not necessarily licensed for cats, nor at the same dose.
• Do not give your animal the leftovers of another animal's prescription — weight, species, health status, and diagnosis are all different.

Your animal swallowed a human medicine — what do you do now?

1. Do not wait for symptoms to appear. Many of these poisons damage the liver or kidney silently before any sign shows; early treatment is far more successful.
2. Do not induce vomiting on your own. Some substances cause greater harm on the way back up, and the animal may aspirate them. Do this only on the direct instruction of a veterinarian.
3. Keep the package or blister strip and take it with you. The active ingredient's name and strength are what determine the treatment plan and the antidote.
4. Record three numbers: the approximate time of ingestion, how many tablets are missing, and the animal's current weight.
5. Call your veterinarian or the nearest emergency clinic immediately, and tell them all of the above before you set off.

Prevention: five habits that spare you the treatment

• Store all medicines — human and veterinary — in a closed, high cabinet; not in a handbag or on a bedside table.
• Do not separate tablets from their original packaging, and keep the package insert.
• Pick up any tablet that falls on the floor immediately — many poisoning cases begin with a single dropped pill.
• Wash your hands after applying any medicated cream, and keep the animal away from the treated area until it dries.
• Keep your veterinarian's number and that of the nearest emergency clinic somewhere visible that every member of the household knows.