A fixed-dose combination analgesic and centrally-acting muscle relaxant. Paracetamol provides analgesic and antipyretic effects by inhibiting central prostaglandin synthesis. Chlorzoxazone acts primarily at the spinal cord and subcortical levels of the brain to inhibit polysynaptic reflexes, relieving muscle spasm and associated pain. This combination is widely used in India for acute, painful musculoskeletal conditions.
Adult: One tablet (Paracetamol 325mg + Chlorzoxazone 250mg) three to four times daily. Maximum: 4 tablets in 24 hours.
Note: Take with or after food to minimize gastric upset. Swallow whole with a full glass of water. Do not crush or chew. The total duration of therapy should generally not exceed 2-3 weeks for the muscle relaxant component.
The combination works synergistically. Paracetamol exerts analgesic and antipyretic effects through central inhibition of prostaglandin synthesis, likely via COX-2 and COX-3 enzyme inhibition, and modulation of the endogenous cannabinoid system. Chlorzoxazone acts primarily at the level of the spinal cord and brainstem by depressing polysynaptic reflex arcs involved in producing and maintaining skeletal muscle spasm, without directly affecting muscle contractility or neuromuscular junction.
Pregnancy: Category C (US FDA). Paracetamol is generally considered safe for short-term use. Chlorzoxazone: Animal studies show adverse effects; no adequate human studies. Use only if potential benefit justifies potential fetal risk, especially in first trimester. Avoid in third trimester due to risk of neonatal sedation.
Driving: May impair mental and/or physical abilities. Drowsiness and dizziness are common. Patients should be advised not to drive or operate heavy machinery until their response to the drug is known.
| Alcohol (Chronic/Heavy Use) | Potentiates hepatotoxicity of paracetamol; increases CNS depression with chlorzoxazone. | Major |
| Warfarin and other Coumarin Anticoagulants | Paracetamol may potentiate anticoagulant effect, increasing INR and bleeding risk, especially with doses >2g/day. | Major |
| Enzyme Inducers (e.g., Carbamazepine, Phenytoin, Rifampicin) | Increase metabolism of paracetamol to toxic NAPQI, increasing hepatotoxicity risk. | Major |
| Other CNS Depressants (e.g., Benzodiazepines, Opioids, Sedating Antihistamines) | Additive sedation, dizziness, and impaired motor skills. | Major |
| Chloramphenicol | Increased half-life of chloramphenicol. | Moderate |
| CYP1A2 Inhibitors (e.g., Ciprofloxacin, Fluvoxamine) | May increase chlorzoxazone levels, increasing sedation risk. | Moderate |
| Metoclopramide, Domperidone | May increase absorption rate of paracetamol. | Minor |
| Isoniazid | May increase risk of paracetamol hepatotoxicity. | Moderate |
Same composition (Paracetamol (325mg) + Chlorzoxazone (250mg)), different brands: