Paracetamol (Acetaminophen)
Paracetamol is a centrally acting analgesic and antipyretic medication used for the control of mild to moderate pain and fever. It has poor anti-inflammatory effects compared to traditional NSAIDs but is believed to produce fewer gastrointestinal side effects, making it a useful alternative for patients with gastric ulceration. Injectable paracetamol is also utilized as an adjunctive perioperative analgesic in dogs. > **CRITICAL WARNING:** Paracetamol is highly toxic and strictly contraindicated in cats due to their inability to safely metabolize the drug. **Clinical Pearl:** The licensed veterinary oral preparation (Pardale-V) contains codeine; however, due to a high first-pass metabolism of opioids in dogs, the codeine component is not bioavailable and does not significantly contribute to analgesia.
Mechanism: The exact mechanism of action remains partially unclear. It is proposed that its antipyretic and analgesic actions are due to: - Decreased **prostaglandin synthesis** specifically within the central nervous system (CNS). - Possible indirect actions on the **cannabinoid system**. Unlike traditional NSAIDs, it has minimal peripheral cyclooxygenase (COX) inhibition, which explains its lack of significant anti-inflammatory activity and reduced gastrointestinal toxicity.
Dosing by species
- Mild to moderate pain / Antipyretic · 10-20 mg/kg · PO/IV · q8h · 5-7 days (for acute pain) · For acute pain management, doses at the top of the range (20 mg/kg) can be used for up to 5-7 days.
- Mild to moderate pain (Pardale-V preparation) · 1 tablet per 12 kg body weight (equivalent to 33 mg/kg) · PO · q8h · Max 7 days before titrating down · Authorized dose for Pardale-V. After 7 days, titrate down to the lowest effective dose. Codeine component is not bioavailable.
- All indications · Do not use · PO/IV · N/A · N/A · Absolute contraindication. Fatal toxicosis due to methaemoglobinaemia.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Cats (absolute contraindication due to fatal toxicosis)
- Patients with pre-existing severe hepatic impairment
Adverse effects
- Hepatotoxicity (dose-dependent)
- Methaemoglobinaemia (especially in cats)
- Facial edema
- Cyanosis
Drug interactions
- Metoclopramide · Enhances the absorption of paracetamol, thereby enhancing its clinical and potentially toxic effects. · moderate
- Cytochrome P450 Inducers (e.g., Phenobarbital, certain NSAIDs) · May increase the production of toxic metabolites (NAPQI), enhancing the risk of hepatotoxicity. · major
Monitoring
- Liver enzyme panel (ALT, AST, ALP, Bilirubin) if prolonged use or overdose is suspected
- Clinical signs of toxicity (jaundice, vomiting, lethargy)
- Mucous membrane color (monitoring for cyanosis/brown blood indicative of methaemoglobinaemia)
Overdose
Overdose of paracetamol causes severe liver damage. Metabolism produces the toxic intermediate **N-acetyl-p-aminobenzoquinonimine (NAPQI)**, which depletes hepatic glutathione, leading to hepatocyte cell death and centrilobular hepatic necrosis. In cats, it causes fatal methaemoglobinaemia. **Treatment:** - Decontamination (emesis/activated charcoal) if ingestion is recent. - **Antidote:** Oral **methionine** or IV **acetylcysteine (N-acetylcysteine)**. These are directed at replenishing hepatic glutathione stores to neutralize NAPQI, though clinical data in veterinary species is limited. - Supportive care (IV fluids, oxygen therapy, blood transfusions if severe methaemoglobinaemia occurs).
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.