ACETAMINOPHEN

Chemistry - A synthetic non-opiate analgesic, acetaminophen (also known as paracetamol occurs as a crystalline, white powder with a slightly bitter taste. It is soluble in boiling water and freely soluble in alcohol. Acetaminophen is known in the U.K. as paracetamol.

Storage, Stability, Compatibility

Acetaminophen products should be stored at temperatures less than 40°C. Do not freeze the oral solution or suspension.

Pharmacology - ACETAMINOPHEN

Acetaminophen produces analgesia and antipyresis via mechanisms similar to the salicylates (inhibition of cyclooxygenase). Unlike aspirin, it does not possess significant antiinflammatory activity.
Uses, Indications - Acetaminophen is occasionally used as an oral analgesic in dogs. In conditions of more severe pain, it may be used in combination with oral codeine phosphate.

Pharmacokinetics - ACETAMINOPHEN

Specific pharmacokinetic information in domestic animals was not located. In humans, acetaminophen is rapidly and nearly completely absorbed from the gut and is rapidly distributed into most tissues. Approximately 25% is plasma protein bound. Dogs apparently exhibit dose dependent metabolism (saturable).

Contraindications, Precautions, Reproductive Safety

Acetaminophen is contraindicated in cats at any dosage. Severe methemoglobinemia, hematuria, and icterus can be seen. Cats apparently are unable to significantly glucuronidate acetaminophen leading to toxic metabolites being formed and resultant toxicity. Dogs also do not metabolize acetaminophen as well as humans and its use must be judicious. In dogs, it is generally not recommended to use acetaminophen during the immediate post-operative phase (first 24 hours) due to an increased risk of hepatotoxicity developing.
Absolute reproductive safety has not been established, but acetaminophen is apparently relatively safe for occasional use in pregnancy (no documented problems in humans). Animal data not located.

Adverse Effects, Warnings

Because acetaminophen is not routinely used in veterinary medicine, experience on its adverse effect profile is limited. At suggested dosages in dogs, there is a potential for renal, hepatic, GI and hematologic effects occurring.
Overdosage - Because of the potentially severe toxicity associated with acetaminophen, consultation with an animal poison center is recommended (see appendix). For overdosage in dogs or cats, standard gut emptying techniques and supportive care should be administered when applicable.
Further treatment with acetylcysteine may be warranted (see acetylcysteine monograph for more information).

Drug Interactions

Large doses may potentiate the effects of coumarin or inandione anticoagulants. Doxorubicin may deplete hepatic glutathione, thereby leading to increased hepatic toxicity. Acetaminophen is not recommended to be used for post-operative analgesia in animals who received halothane anesthesia. Chronic use of acetaminophen in combination with other analgesics may lead to renal pathologies.
Laboratory Considerations - False positive results may occur for urinary 5-hydroxyindoleacetic acid.