1. Clinical Overview
Paracetamol (Acetaminophen) 1000mg is a widely used, centrally-acting, non-opioid analgesic and antipyretic agent. It is a first-line treatment for mild to moderate pain and fever. Unlike NSAIDs, it has minimal anti-inflammatory activity and a favorable gastrointestinal safety profile. The 1000mg strength is a standard adult single dose, often used for more significant pain or as a single daily dose in certain extended-release formulations.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: 30-60 minutes. Peak plasma concentration reached in 0.5-2 hours. | Approximately 4-6 hours for immediate-release formulations. | Approximately 85-98% after oral administration. |
2. Mechanism of Action
The exact mechanism is not fully elucidated but is distinct from NSAIDs. It is a potent inhibitor of cyclooxygenase (COX) enzymes, particularly in the central nervous system (CNS). It is believed to act primarily on the COX-2 variant and a putative central COX-3 enzyme. It reduces the production of prostaglandins in the brain (hypothalamus), which mediates its antipyretic effect and central analgesic action. It has minimal effect on peripheral COX-1, accounting for its lack of significant anti-inflammatory activity and GI side effects.
3. Indications & Uses
- Mild to moderate pain (e.g., headache, toothache, musculoskeletal pain, dysmenorrhea)
- Fever (pyrexia) of any etiology
4. Dosage & Administration
Adult Dosage: 1000mg every 4-6 hours as needed for pain/fever. Maximum daily dose: 4000mg (4 grams). Do not exceed 8 tablets/capsules of 500mg or 4 tablets of 1000mg in 24 hours.
Administration: Take with or without food. Swallow the tablet whole with a full glass of water. Do not crush or chew unless specified (e.g., dispersible tablets). For faster absorption, take on an empty stomach. Do not take for more than 3 days for fever or 5 days for pain without consulting a doctor.
5. Side Effects
Common side effects may include:
- Nausea
- Epigastric discomfort/pain
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Warfarin and other Coumarin Anticoagulants | Increased INR and risk of bleeding, especially with chronic high-dose paracetamol (>2g/day for several days). | Major |
| Isoniazid | Increased risk of hepatotoxicity due to induction of CYP2E1, increasing NAPQI formation. | Major |
| Enzyme Inducers (Phenobarbital, Phenytoin, Carbamazepine, Rifampicin) | Increased metabolism to toxic NAPQI, elevating hepatotoxicity risk. | Major |
| Probenecid | Decreases paracetamol conjugation, may increase its half-life and effect. | Moderate |
| Metoclopramide, Domperidone | Increased absorption rate of paracetamol. | Minor |
| Cholestyramine | Decreased absorption of paracetamol if taken within 1 hour. | Moderate |
| Alcohol (Chronic, Heavy Use) | Induces CYP2E1, depletes glutathione, significantly increasing risk of hepatotoxicity even at therapeutic doses. | Major |
| Other Hepatotoxic Drugs (e.g., Methotrexate, Azathioprine, Ketoconazole) | Additive risk of liver damage. | Major |
7. Patient Counselling
- DO take exactly as directed. Do not exceed 4000mg (4g) in 24 hours.
- DO check all other medication labels (cold, flu, combination analgesics) for 'paracetamol' or 'acetaminophen' to avoid accidental overdose.
- DO NOT take for more than 3 days for fever or 5 days for pain without medical advice.
- DO NOT consume alcohol while taking this medicine.
- DO NOT take if you have severe liver disease.
- DO inform your doctor if you are taking warfarin.
8. Toxicology & Storage
Overdose: Stage 1 (0-24h): Nausea, vomiting, anorexia, malaise, pallor, diaphoresis. Stage 2 (24-72h): Right upper quadrant pain, elevated liver enzymes (AST/ALT), prolonged PT/INR. Patient may appear to improve clinically. Stage 3 (72-96h): Peak hepatotoxicity - jaundice, coagulopathy, hypoglycemia, hepatic encephalopathy, acute liver failure, renal failure (oliguria). Stage 4 (4 days-2 weeks): Recovery or death from multi-organ failure.
Storage: Store below 30°C, in a cool, dry place. Protect from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.