Paracetamol (Acetaminophen)

Paracetamol is a centrally acting analgesic and antipyretic with little useful peripheral anti-inflammatory activity. In veterinary practice it is principally an option for selected canine patients.

It must not be administered to cats. Equine and calf regimens have been reported, but food-animal residue requirements are not established by the dose alone.

During prolonged treatment, assess hepatic and renal function and the CBC, particularly when anorexia, vomiting, lethargy or abnormal mucous-membrane color develops.

Group

Non-opioid analgesic and antipyretic

Mechanism of Action

Analgesia involves central prostaglandin modulation and descending pain pathways. The precise contribution of serotonergic and other central mechanisms remains incompletely defined; selective COX-3 inhibition is not an established complete explanation.

Dose Rates

Dogs

Horses

Cattle

Indications

• Mild pain or adjunctive multimodal analgesia in dogs.
• Selected equine pain management.
• Reported antipyretic use in calves.

Contraindications

• Cats: severe oxidative erythrocyte injury and hepatic toxicity can occur.
• Hepatic disease or concurrent hepatotoxic exposure increases concern.
• Avoid duplicate paracetamol-containing products and unassessed combination cold or analgesic preparations.
• Do not administer to ferrets. Avoid use in sugar gliders or hedgehogs because safety is not established.
• Reproductive safety in veterinary patients is not well established; do not infer safety from human pregnancy classifications.

Interactions

• Enzyme-modifying or hepatotoxic drugs may increase toxicity; the magnitude of many interactions is not established in animals.
• Adding codeine does not guarantee improved canine analgesia.
• Barbiturates and isoniazid can increase concern about formation of hepatotoxic metabolites.
• Doxorubicin can deplete hepatic glutathione and may increase hepatic injury.
• Avoid paracetamol for postoperative analgesia following halothane exposure because of hepatic toxicity concerns.
• Large or repeated doses can enhance warfarin anticoagulant effects; chronic combinations with other analgesics can increase renal injury risk.
• Phenothiazines may increase hypothermia risk.

Adverse effects

• Hepatic injury, especially with excessive or repeated exposure.
• Methemoglobinemia, Heinz body anemia, facial or paw edema and hypoxia in toxicosis.
• Vomiting, lethargy and anorexia with intoxication.
• Keratoconjunctivitis sicca has been reported with canine poisoning.

Toxicity

Reactive metabolites cause oxidative erythrocyte damage and hepatic necrosis when detoxification capacity is exceeded. Cats are particularly susceptible. Suspected poisoning requires prompt acetylcysteine and supportive treatment, with monitoring of red cells, methemoglobin and hepatic function.

Preparations