FrançaisMy dog or cat ate Tylenol: what to do
Call your vet or a pet poison line now, even if your pet seems fine. Acetaminophen is the active drug in Tylenol, also called paracetamol; it is safe for people but has a narrow margin of safety in dogs and is especially dangerous for cats [13]. Signs can progress quickly [14], and the sooner treatment starts, the better the outlook [1, 2]. Keep the box so the vet knows the product, the strength and how many tablets are missing.
Do not give anything at home and do not wait for signs. Human medicines are among the most common causes of calls to the ASPCA Animal Poison Control Center for both dogs and cats [18]. In a Canadian teaching hospital, acetaminophen was the second most common medicine swallowed by dogs, in 29 of 223 cases [12]. In cats, it is one of the most frequent causes of poisoning [5], and the most common cause in one case series was the owner giving the tablet [2]. Acetaminophen is in the same family of risks as ibuprofen and other toxic household items.
Why cats are so sensitive to acetaminophen
Acetaminophen is absorbed fast from the gut: it usually peaks in the blood within 1 hour, later with extended-release products [1]. The liver normally clears it through two routes, called glucuronidation and sulfation [1].
Cats have little of the enzyme needed for glucuronidation. They rely on sulfation, and when that route is full, toxic breakdown products form [1]. In a study giving single doses to dogs and cats, cats reached similar effects at much lower doses than dogs, kept the drug longer in the blood, and showed signs of a saturated sulfation route [3].
In dogs and cats, the main harm from an overdose is to red blood cells: methemoglobin and hemolysis [6]. A breakdown product called para-aminophenol appears to cause the methemoglobin, and both species lack enough of an enzyme (N-acetyltransferase) that would protect them [6]. In short: red blood cells are the main target in cats, the liver in dogs [14].
Signs and timing in cats and dogs
Methemoglobin is hemoglobin whose iron has been oxidized, so it can no longer carry oxygen. It gives a brown color to the blood plasma and the gums [7, 8].
| Cats | Dogs | |
|---|---|---|
| Main damage | Red blood cells (methemoglobin, then Heinz bodies); liver with large amounts; kidney injury possible [1] | Liver more often than in cats; methemoglobin at higher amounts [1] |
| Timing | Methemoglobin within a few hours, Heinz bodies after [1] | Liver values rise over the next 1 to 2 days; the vet rechecks them at 24 and 48 hours [1, 9] |
| Gums and breathing | Brown or muddy gums, fast heart, fast or labored breathing, bluish color [1, 2] | Fast heart and fast breathing [1] |
| Swelling | Swollen face or paws [1] | Swollen face or paws [1, 15] |
| Other signs | Dullness, weakness, not eating, vomiting, low body temperature; yellow gums (jaundice) with large amounts [1, 2] | Not eating, belly pain, vomiting, dullness, trembling, jaundice; sometimes dry eye [1] |
Dark urine can appear when red cells burst inside the vessels and hemoglobin passes into the urine, as reported in a Dalmatian [15]. Like onions and garlic, acetaminophen is an oxidant that can cause Heinz body anemia [7, 8].
How much is dangerous, and where it hides
In cats, poisoning generally occurs from 40 to 50 mg/kg, but some cats have shown signs at 10 mg/kg [1]. In a series of 17 cats, cats that died had received from 10 to 170 mg/kg [2]. In dogs, signs are reported mainly above 100 mg/kg and methemoglobin problems above 200 mg/kg, and repeated doses can cause poisoning at lower amounts [1]. Call your vet or a pet poison line whatever the amount.
Acetaminophen is also in many cough, cold, flu, allergy and fever products, and in some prescription painkillers, sometimes combined with tramadol [1, 13]. Brand names include Tylenol, Panadol, Excedrin and Percocet [13]. Check the label of any medicine your pet got into.
What the tests show, and why
The vet looks at the gums, the color of the blood and the breathing, and follows the red cells and the liver over the next days. Pets with acetaminophen poisoning are monitored often for methemoglobin, Heinz body anemia and hemolysis, and liver enzymes are rechecked at 24 and 48 hours [1].
- Abbreviation ALT, sometimes written ALAT or SGPT, in the liver block of the chemistry panel.
- Result 640 U/L in this illustrative dog line: well above the interval. Liver damage is more common with acetaminophen in dogs than in cats [1].
- Unit U/L: units of enzyme activity per liter of serum.
- Laboratory interval The example uses the dog interval of the Merck Veterinary Manual (cat: 25–97 U/L) [11]; each laboratory sets its own.
- Flag H: above the interval. After acetaminophen the vet rechecks liver enzymes at 24 and 48 hours [1], because ALT peaks around 48 hours after an acute injury [9].
| Test | What can be found | Why it matters |
|---|---|---|
| Methemoglobin | Raised; brown plasma | In cats given acetaminophen experimentally, methemoglobin rose to 21.7% and 45.5% of hemoglobin at the two higher doses [4] |
| Blood smear | Heinz bodies and eccentrocytes; Heinz bodies show best with new methylene blue stain [7] | They mark oxidant damage. Healthy cats can have a few small Heinz bodies, so the vet judges the number and size [7] |
| Hematocrit (PCV) | Falls as red cells are destroyed; the anemia is usually regenerative [7] | Shows how much oxygen-carrying blood is left; some pets need a transfusion [1]. See hemolytic anemia in cats |
| Hemoglobin | May read falsely high when there are many Heinz bodies [17] | The vet reads it together with the hematocrit [17] |
| Urine | Red-brown urine (hemoglobinuria) in some pets [7, 15] | Sign that red cells burst inside the vessels [7] |
| ALT | High, especially in dogs | Liver cell damage; after an acute injury ALT peaks around 48 hours [9] |
| Bilirubin | High | From red cell breakdown, liver damage or both; jaundice shows above about 3 mg/dL [10]. See bilirubin and GGT |
| Kidney values | Sometimes high in cats | Acute kidney injury is possible in cats [1] |
Monitoring continues after the first day: in cats given a high dose experimentally, a red-cell protector called glutathione dropped within 24 hours and took up to 8 days (192 hours) to return to normal [4]. For a cat, the anemia part is explained in anemia in cats.
Care and outlook
Treatment aims to remove the drug early, limit methemoglobin and liver damage, and support the pet [1]. If the ingestion was recent, the vet may make the pet vomit and give activated charcoal [1]. The vet may also give an antidote, acetylcysteine, which can reduce methemoglobin and liver damage [1]; a 2026 review found its strongest evidence in early use for acetaminophen poisoning in cats and dogs [16]. Hospital care can include fluids and blood transfusions [1].
The outlook depends on the amount swallowed and how soon treatment starts [1]. In the series of 17 cats, 12 survived; 10 of them were treated within 14 hours, and the authors thought the delay before treatment could matter as much as the dose [2]. In the Canadian series of dogs that swallowed medicines of all kinds, 99.6% survived to discharge [12].
Related articles
Other poisons that damage red cells or the liver: onion and garlic toxicity. In cats, liver values are explained in high liver enzymes in cats.
En français : paracétamol, chien et chat.
Questions to ask your veterinarian
- What was my pet’s methemoglobin level, and when will it be measured again?
- Are there Heinz bodies on the blood smear, and how low is the hematocrit?
- Will a transfusion be needed?
- Are ALT and bilirubin normal, and will they be rechecked at 24 and 48 hours?
- Should the kidney values be checked too?
Get your pet’s emergency blood results explained.
AI DiagAnimals reads the hematocrit, liver enzymes, bilirubin and kidney values together with your laboratory’s intervals, and prepares the questions for your vet. The service opens soon: leave your email to be told first.
Frequently asked questions
How fast does Tylenol poisoning happen in dogs?
Acetaminophen is absorbed fast and usually peaks in the blood within 1 hour [1]. Liver damage is the main risk in dogs and shows on blood work over the following days, so the vet rechecks liver enzymes at 24 and 48 hours [1]. Call your vet or a pet poison line as soon as you know your dog swallowed it.
What happens if a cat eats a 500mg Tylenol?
It is an emergency: call your vet or a pet poison line now. Cats are especially sensitive [13]; some have shown signs at 10 mg/kg, and poisoning generally occurs from 40 to 50 mg/kg [1]. Their red blood cells are damaged within a few hours [1]. Call your vet or a pet poison line whatever the amount.
How long does it take for Tylenol to affect a cat?
Methemoglobin builds up within a few hours, with brown gums, fast breathing and weakness; Heinz bodies follow [1]. Do not wait for signs: in a series of 17 cats, most survivors were treated within 14 hours [2].
Sources
- Hovda T. Toxicoses From Human Analgesics in Animals. Merck Veterinary Manual, reviewed 2024, updated 2025. merckvetmanual.com/toxicology/toxicoses-from-human-analgesics/toxicoses-from-human-analgesics-in-animals
- Aronson LR, Drobatz K. Acetaminophen toxicosis in 17 cats. J Vet Emerg Crit Care. 1996;6(2):65-69. doi.org/10.1111/j.1476-4431.1996.tb00034.x
- Savides MC, Oehme FW, Nash SL, Leipold HW. The toxicity and biotransformation of single doses of acetaminophen in dogs and cats. Toxicol Appl Pharmacol. 1984;74(1):26-34. doi.org/10.1016/0041-008x(84)90266-7
- Nash SL, Savides MC, Oehme FW, Johnson DE. The effect of acetaminophen on methemoglobin and blood glutathione parameters in the cat. Toxicology. 1984;31(3-4):329-334. doi.org/10.1016/0300-483x(84)90114-8
- Rumbeiha WK, Lin YS, Oehme FW. Comparison of N-acetylcysteine and methylene blue, alone or in combination, for treatment of acetaminophen toxicosis in cats. Am J Vet Res. 1995;56(11):1529-1533. pubmed.ncbi.nlm.nih.gov/8585668
- McConkey SE, Grant DM, Cribb AE. The role of para-aminophenol in acetaminophen-induced methemoglobinemia in dogs and cats. J Vet Pharmacol Ther. 2009;32(6):585-595. doi.org/10.1111/j.1365-2885.2009.01080.x
- eClinPath (Cornell University). Red blood cell inclusions: Heinz bodies. eclinpath.com/hematology/morphologic-features/red-blood-cells/rbc-inclusions
- eClinPath (Cornell University). Causes of anemia: oxidant injury. eclinpath.com/hematology/anemia/causes-of-anemia
- eClinPath (Cornell University). Alanine aminotransferase. eclinpath.com/chemistry/liver/liver-injury/alanine-aminotransferase
- eClinPath (Cornell University). Bilirubin. eclinpath.com/chemistry/liver/cholestasis/bilirubin
- Merck Veterinary Manual. Serum biochemical analysis reference ranges. merckvetmanual.com/reference-values-and-conversion-tables/reference-guides/serum-biochemical-analysis-reference-ranges
- Kennedy J et al. Pharmaceutical exposure and toxicosis in dogs: a retrospective study of 223 cases from a Canadian veterinary teaching hospital (2018 to 2023). Can Vet J. 2025;66(11):1218-1229. pubmed.ncbi.nlm.nih.gov/41142969
- Pet Poison Helpline. Acetaminophen. petpoisonhelpline.com/poison/acetaminophen
- Villar D, Buck WB, Gonzalez JM. Ibuprofen, aspirin and acetaminophen toxicosis and treatment in dogs and cats. Vet Hum Toxicol. 1998;40(3):156-162. pubmed.ncbi.nlm.nih.gov/9610496
- MacNaughton SM. Acetaminophen toxicosis in a Dalmatian. Can Vet J. 2003;44(2):142-144. pubmed.ncbi.nlm.nih.gov/12650044
- Tóth ZT et al. From antidote to multimodal adjunct: a systematic review of N-acetylcysteine in dogs and cats. Front Vet Sci. 2026;13:1959626. doi.org/10.3389/fvets.2026.1959626
- eClinPath (Cornell University). Hemoglobin. eclinpath.com/hematology/tests/hemoglobin
- Swirski AL et al. Companion animal exposures to potentially poisonous substances reported to a national poison control center in the United States in 2005 through 2014. J Am Vet Med Assoc. 2020;257(5):517-530. doi.org/10.2460/javma.257.5.517
Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.