Acetominaphen Toxicity in Dogs and Cats, Part 1—Symptoms

By Carl Palazzolo, DVM, Long Beach Animal Hospital (LBAH) 

Photo by Laura.

 In a recent article, Dr. Palazzolo warned pet owners about the dangers of acetaminophen toxicity. People still wonder whether certain doses are safe for dogs, cats or both; he’s expanding the first article into two parts. 

If you have a cold or a headache, you may take an over-the-counter acetaminophen product like Tylenol, Excedrin or Nyquil, in which the anti-inflammatory drug is mixed with other ingredients. If you don’t want to have a bigger headache, keep the stuff away from the dog and especially the cat.

Acetaminophen actually can be given to dogs when used at a proper dose, although there are many new anti-inflammatory drugs used in dogs that are more effective and have fewer side effects, so we don’t recommend a dose of acetaminophen for the dog anymore. But people still use it, so we want to offer a few facts and cautions.

The primary source of toxicity in dogs and cats is overdose from chewing into bottles that contain this drug, or finding pills on the ground and ingesting enough to cause liver failure. Just one of those little regular-strength (325mg) tablets is enough to cause a problem.

If you have a bottle somewhere in the house and your pet exhibits the following symptoms, don’t even bother checking the bottle. Call the vet immediately—death can follow these symptoms.

Acetaminophen Poisoning Symptoms:

  • Lethargy, depression or coma
  • Vomiting or diarrhea
  • Swelling of the face or paws
  • Difficulty breathing
  • Brown or yellow gums
  • Brown or dark urine

If the acetaminophen your pet ingested is combined with another drug that contains caffeine or antihistamines, there are other symptoms that might occur:

  • Ataxia, a loss of muscle coordination that makes the animal walk like the proverbial drunken sailor)
  • Agitation
  • Tremors
  • Seizures

A tentative diagnosis is made on a pet with the above symptoms by running a complete blood panel and urinalysis. There will be clues as to this toxicity in these tests, one of which is an elevation in liver enzymes and another is a type of red blood cell called a Heinz body. These clues, although they help point to a diagnosis, do not give a definitive diagnosis.

Liver failure from acetaminophen poisoning can show up on a blood panel. All of these high tests are showing a major liver problem. Photo courtesy of LBAH.

A measure of the oxygen saturation of the red blood cells can be determined using a pulse oximeter.

The pulse oximeter gives the percentage of oxygen saturation. Photo courtesy of LBAH.

An acetaminophen toxicity test needs to be run to make the final diagnosis. This is a specialized test that takes time to come back from the lab, so when we have a pet with suspected acetaminophen toxicity, we treat for it as a precaution.

And cat people, listen big: Cats are very sensitive to acetaminophen toxicity, so drugs that contain them are never given to cats. They get a specific form of toxicity called methemoglobinemia, a condition that destroys red blood cells and causes liver problems. Cats’ physical composition is deficient in an enzyme called glucuronyl transferase, which metabolizes acetaminophen.


Pets with methemoglobinemia need supplemental oxygen. Photo courtesy of LBAH.

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