1. Clinical Overview
Paracetamol (Acetaminophen) 650mg 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. In the Indian context, it is one of the most commonly consumed over-the-counter (OTC) and prescribed medicines, available in numerous single and combination formulations.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: 30-60 minutes for analgesic effect; 30 minutes for antipyretic effect. | Approximately 4-6 hours. | Approximately 85-98% after oral administration. |
2. Mechanism of Action
The exact mechanism is not fully elucidated but is distinct from NSAIDs. It acts primarily centrally by inhibiting prostaglandin synthesis in the central nervous system (CNS). It is a weak inhibitor of peripheral cyclooxygenase (COX) enzymes, particularly COX-2, with minimal effect on COX-1, explaining its lack of significant anti-inflammatory or antiplatelet activity and its low gastrointestinal toxicity.
3. Indications & Uses
- Mild to moderate pain (e.g., headache, toothache, musculoskeletal pain, dysmenorrhea)
- Fever (pyrexia) of any etiology
4. Dosage & Administration
Adult Dosage: 650mg every 4-6 hours as needed for pain or fever. Do not exceed 4 grams (4000mg) in 24 hours. A lower maximum of 3 grams (3000mg) per day is recommended for long-term use, in the elderly, or in those with risk factors for hepatotoxicity.
Administration: May be taken with or without food. Taking with food may slightly delay absorption. Swallow tablet whole with a full glass of water. Do not crush, chew, or break extended-release formulations. For rapid relief, take on an empty stomach. Do not use 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 | Chronic high-dose paracetamol (>2g/day for several days) may potentiate anticoagulant effect, increasing INR and risk of bleeding. | Major |
| Isoniazid | Increases the formation of the toxic metabolite NAPQI via CYP2E1 induction, significantly raising risk of hepatotoxicity, even at therapeutic doses. | Major |
| Carbamazepine, Phenytoin, Phenobarbital | Enzyme inducers increase NAPQI formation, increasing hepatotoxicity risk. Dose reduction of paracetamol may be needed. | Moderate |
| Alcohol (Chronic, heavy use) | Induces CYP2E1 and depletes glutathione, dramatically increasing risk of severe and potentially fatal hepatotoxicity with therapeutic or supratherapeutic doses. | Major |
| Probenecid | May decrease the metabolic clearance of paracetamol, leading to increased plasma levels. Dose reduction may be necessary. | Moderate |
| Metoclopramide, Domperidone | May increase the absorption rate of paracetamol. | Minor |
| Cholestyramine | Reduces absorption of paracetamol if taken within 1 hour. Administer at least 1 hour before or 4-6 hours after cholestyramine. | Moderate |
7. Patient Counselling
- DO take the exact dose as prescribed or as per label instructions.
- DO NOT exceed 4 grams (8 tablets of 500mg/6 tablets of 650mg) in 24 hours.
- DO NOT take for more than 3 days for fever or 5 days for pain without medical advice.
- DO check all other medicines (cold, flu, combination analgesics) for paracetamol content to avoid accidental overdose.
- DO inform your doctor if you consume 3 or more alcoholic drinks daily.
- DO store the medicine out of reach of children.
8. Toxicology & Storage
Overdose: Stage 1 (0-24 hrs): Nausea, vomiting, anorexia, malaise, pallor, diaphoresis. Stage 2 (24-72 hrs): Resolution of initial symptoms; right upper quadrant pain/tenderness; elevated liver enzymes (AST/ALT), bilirubin, PT/INR. Stage 3 (72-96 hrs): Peak hepatotoxicity: jaundice, coagulopathy, hypoglycemia, hepatic encephalopathy, acute liver failure, renal failure (oliguria), metabolic acidosis, coma, death. Stage 4 (4 days-2 weeks): Recovery in survivors.
Storage: Store below 30°C (86°F). Protect from light and moisture. Keep in the original blister pack or container. Do not store in bathroom cabinets. Keep out of reach and sight of children.