Acetaminophen
Tylenol | Panadol
Overview
Acetaminophen is a widely used non-opioid analgesic and antipyretic for the relief of mild to moderate pain and fever. It has little anti-inflammatory activity.
Indications
- Mild to moderate pain
- Fever
Contraindications
- Severe hepatic impairment or active liver disease
- Hypersensitivity to acetaminophen
Classification
Mechanism of Action
The exact mechanism is not fully established but is thought to involve central inhibition of prostaglandin synthesis and modulation of the endocannabinoid and serotonergic systems, producing analgesia and antipyresis.
Pharmacodynamics
Reduces pain and fever with minimal peripheral anti-inflammatory or antiplatelet effect, and without significant gastric irritation at therapeutic doses.
Pharmacokinetics
- Absorption
- Rapidly and almost completely absorbed from the GI tract.
- Distribution
- Widely distributed; relatively low protein binding at therapeutic doses.
- Metabolism
- Metabolized in the liver mainly by glucuronidation and sulfation; a small fraction forms the toxic metabolite NAPQI via CYP2E1.
- Excretion
- Excreted in urine, predominantly as glucuronide and sulfate conjugates.
- Half-life
- 2-3 hours
- Bioavailability
- 70-90%
- Protein Binding
- Approximately 10-25%
Dosage
Typical dosage: 325-1000 mg every 4-6 hours; maximum 4 g per day
Available Forms
- Tablet
- Capsule
- Oral suspension
- Suppository
- Injection
Side Effects
Common
- Nausea
Serious
- Hepatotoxicity with overdose
Rare
- Stevens-Johnson syndrome
- Toxic epidermal necrolysis
- Acute generalized exanthematous pustulosis
Drug Interactions
Regular high-dose acetaminophen may enhance the anticoagulant effect and raise INR.
Chronic alcohol use increases the risk of acetaminophen-induced hepatotoxicity.
Warnings
Pregnancy
Category B
Toxicity
Overdose causes dose-dependent hepatic necrosis due to accumulation of the reactive metabolite NAPQI.
Overdose
Treat with N-acetylcysteine, guided by the acetaminophen level and the Rumack-Matthew nomogram; give activated charcoal if early.
References
Looking for patient-friendly information? Visit RemedyDoor for easy-to-read guides about this medication.