Paracetamol poisoning in dogs

Overview

Paracetamol and its effects on dogs

Paracetamol is an active ingredient in human painkillers and fever medicines. Packaging may use the alternative English name acetaminophen. Cold remedies can contain it even when another use is emphasised on the pack.

Poisoning can convert haemoglobin into methaemoglobin, impairing oxygen transport. Red cell destruction can also reduce circulating cell numbers. Paracetamol metabolites can damage liver cells too.

Human preparations include tablets, capsules, liquids and suppositories. Strengths vary, so tablet numbers alone cannot establish exposure without product details.

Causes

Absorption and liver injury

Paracetamol is rapidly absorbed from the digestive tract. Standard preparations usually reach peak plasma concentrations within one hour. Extended-release tablets can be absorbed more slowly, delaying the peak.

The liver converts some paracetamol into water-soluble forms excreted through the kidneys. Some becomes a reactive metabolite called NAPQI. Glutathione normally helps bind it before liver cells are damaged. During overdose, liver glutathione stores may be insufficient.

Accumulating harmful metabolite can bind proteins within liver cell structures. Severe injury kills cells, particularly in the centres of liver lobules.

Oxidative red cell injury

Oxidative blood injury involves para-aminophenol, normally formed in small amounts from paracetamol. Dogs lack enzymes used by many other animals to eliminate it. Oxidative injury can form Heinz bodies, deposits of precipitated haemoglobin within red cells.

Canine red cells may have limited capacity to convert methaemoglobin back into functioning haemoglobin.

Risk factors

Exposure can follow access to unattended packaging or administration of human painkillers. Repeated administration can cause poisoning with smaller individual amounts. Delayed absorption of several tablets can prolong high blood concentrations and increase liver injury risk. Exposure assessment needs body weight, amount swallowed and product strength.

At a Canadian veterinary hospital, paracetamol was involved in 29 of 223 canine pharmaceutical exposures. Owners had deliberately administered it in two of these 29 cases.

Hospital exposures were recorded during 2018–2023. Ingestion quantity could be estimated for 14 paracetamol-exposed dogs. They were examined between 20 minutes and 24 hours after exposure. None of these 14 had abnormal clinical examination findings.

Combination products may contain another active ingredient such as tramadol. In one dog roaming freely in a park, symptom onset raised suspicion and serum testing confirmed paracetamol.

Typical symptoms

Methaemoglobin-associated respiratory signs have been described within four to 12 hours of exposure.

• Breathing can accelerate as tissues receive less oxygen.
• Vomiting or apparent abdominal pain can occur.
• Appetite may decrease.
• Dogs may be unusually subdued or tremble.
• The area around the eyes, muzzle or paws may swell.
• Heart rate can increase.
• Urine can turn brown.

Liver injury can cause inappetence, vomiting or diarrhoea within 24 hours. Yellow gums or eye whites can indicate liver injury.

Obvious signs of liver cell death can start at around 36 hours once injury has progressed.

In one case, respiratory rate reached 50 breaths per minute at the clinic. The same dog had conjunctival swelling in both eyes.

Complications and consequences

Oxygenation, liver and kidneys

Severe methaemoglobinaemia can deprive tissues of oxygen despite faster breathing. Severe poisoning can impair liver production of clotting factors. Rapidly progressing liver injury can raise ammonia at 36–48 hours. At worst, neurological signs can progress to coma or death within 72 hours. Paracetamol can also damage kidney tubules.

Some dogs have developed sudden dry eye after ingestion.

Ammonia accumulating in severe liver injury can affect brain function. Albumin can fall as the damaged liver produces less of this protein. Intravascular red cell destruction can release haemoglobin into urine. In a severe clinical case, post-mortem examination found haemoglobin casts in kidney tubules.

Severe clotting dysfunction can cause bleeding in several organs. Liver examination in the same dog also showed impaired bile flow.

Diagnosis at the vet

Exposure details and blood samples

The vet assesses exposure details alongside symptoms and blood results. Methaemoglobin can also turn a blood sample brown. Smear examination can reveal oxidative injury as Heinz bodies. Initial haematocrit can be within range despite anaemia developing over subsequent days. Liver enzymes are rechecked at 24 and 48 hours for injury.

A co-oximeter uses light to distinguish haemoglobin forms and measure methaemoglobin. Raised methaemoglobin does not identify the cause: mild increases also occur in dogs with sepsis, for example. Bilirubin can rise with liver injury.

Progression of anaemia and liver injury

A five-year-old Australian Cattle Dog had 25.6% methaemoglobin at admission in one case. Initial haematocrit, the red cell volume fraction, was 0.48 l/l. Two days later it was 0.23 l/l. Heinz bodies then occurred in around 80% of red cells. ALT activity, indicating liver cell injury, peaked on hospital day two.

Peak ALT exceeded 10,844 U/l. It was 4,503 U/l the following hospital day. Bilirubin was then 203 µmol/l. Serum paracetamol was confirmed using liquid chromatography and mass spectrometry.

Other similar findings

Similar oxidative red cell injury can be suspected after onion or garlic ingestion. If eyes become dry, tear production can be assessed with a Schirmer tear test.

Treatment

Stopping exposure and giving the antidote

After recent exposure, the vet may induce vomiting to remove unabsorbed medicine. This is unsuitable without a swallowing reflex because stomach contents could enter the airway. Activated charcoal may reduce absorption once vomiting has stopped.

N-acetylcysteine is the antidote used to limit blood and liver injury. It helps form more glutathione, protecting liver cells from reactive metabolites. It can also bind paracetamol metabolites directly and promote elimination.

Supportive care and monitoring

A hypoxic dog may need oxygen in hospital. Severe anaemia can prompt consideration of red cell transfusion. Blood typing beforehand helps select a suitable product. Intravenous fluids can treat vomiting-related dehydration. Antiemetics can control vomiting. Repeat samples monitor methaemoglobin and red cell destruction during treatment.

Recovery depends partly on the delay between exposure and treatment. The amount swallowed also affects prognosis.

Clinical follow-up also monitors kidneys because injury can emerge within 24–72 hours of ingestion. Fluids can correct electrolyte disturbances caused by vomiting. Haematocrit after red cell transfusion shows how much the red cell proportion increased.

Discharge and recovery

All 29 paracetamol-exposed dogs at the Canadian hospital survived to discharge. An 11-year-old Dalmatian treated for severe oxygen transport impairment was discharged on day four. Mild anaemia remained at discharge. Liver values were within reference intervals. A Dachshund was discharged after three and a half days in hospital. It still needed dry eye medication after discharge.

Prevention

Keep human medicines in a closed, inaccessible cupboard. Use tightly closed containers during travel to prevent chewing of packaging. Do not leave medicine-containing bags open on the floor. Check cold remedy ingredients before storing them so you know whether paracetamol is present. Do not give human painkillers on your own initiative.

Keep a record of packed medicines during travel so missing products can be identified quickly.

Important for dog owners

Prevent further access to medicines. Bring packaging or a photograph to the appointment. Report the product name, strength and estimated missing tablet count by phone. Also give estimated exposure time and body weight. Mention any other active ingredients. If medicine was given repeatedly, report every occasion. Do not give other medicines or home remedies before veterinary advice.

Call an emergency veterinary service if your regular practice is closed. Report canine exposure to a vet.

Contact a vet after finding chewed packaging even if the swallowed amount cannot be established.

Check other substances that can poison dogs.

Frequently asked questions

Can dogs have paracetamol for pain?

Only give it if a vet has prescribed it for that individual dog. Contact a vet immediately after accidental administration above the prescribed amount. Repeated excess amounts can cause poisoning with smaller individual ingestions.

Can I monitor a dog without symptoms at home?

First call a vet immediately after suspected ingestion. Early treatment can reduce the amount remaining unabsorbed. Exposure details guide whether immediate treatment or later liver blood tests are needed.

Why might gums look brown or bluish?

The colour can reflect increased methaemoglobin. This impairs oxygen delivery from blood to tissues. Seek emergency care immediately for brown or bluish gums after suspected exposure.

Why are blood tests repeated?

The initial red cell proportion can be normal before falling over subsequent days. Liver enzymes are rechecked at 24 and 48 hours. Later samples also look for red cell destruction or raised methaemoglobin.

What if my dog ate a paracetamol-containing cold remedy?

Check all active ingredients and report them to the vet. Other ingredients alongside paracetamol can influence treatment choice. Packaging helps establish what and how much was swallowed.