What is the time of onset of acetaminol (paracetamol)?

Time of Onset of Paracetamol (Acetaminophen)

Paracetamol begins to provide pain relief within 15-30 minutes after oral administration, with peak plasma concentrations occurring at 60-120 minutes. 1

Pharmacokinetic Profile

Paracetamol (acetaminophen) has several key characteristics that influence its onset of action:

• Absorption rate: Paracetamol is rapidly absorbed from the gastrointestinal tract following oral administration 2
• Bioavailability: Systemic bioavailability ranges from 70-90% after oral administration 2
• Distribution: It distributes rapidly and evenly throughout most tissues and fluids 2
• Time to effect: Clinical evidence shows that pain relief begins at approximately 15-30 minutes after ingestion 1, 3

Factors Affecting Onset of Action

Several factors can influence how quickly paracetamol takes effect:

• Food intake: Gastric emptying is delayed by food, which can slow absorption 2
• Formulation type: Effervescent tablets provide more rapid absorption and onset of action compared to conventional tablets 4
• Route of administration:
  • Oral: 15-30 minutes to onset 1
  • Rectal: Slower and more irregular absorption compared to oral route 4
  • Intravenous: Most rapid onset (when available in this form) 4

Clinical Evidence

A double-blind, placebo-controlled study specifically examining onset of analgesia found that paracetamol tablets containing sodium bicarbonate demonstrated significant pain relief beginning at 15 minutes post-dose compared to placebo 3. This provides strong evidence for the rapid onset of action of paracetamol.

Dosing Considerations

For optimal effect in adults:

• Optimal unit dose: 1 gram 4
• Maximum daily dose: 4 grams 4
• Duration of analgesic effect: Approximately 4-6 hours 4

For children:

• Recommended dose: 15 mg/kg every 4-6 hours 4, 5
• Maximum daily dose: 60 mg/kg/day 4

Common Pitfalls and Caveats

1. No direct correlation: There is no direct correlation between serum concentrations of paracetamol and its analgesic or antipyretic effect 4

2. Hepatotoxicity risk: While paracetamol is generally safe at therapeutic doses, exceeding the maximum recommended daily dose can cause severe hepatic necrosis 2

3. Delayed absorption: Food intake and certain medications (propantheline, pethidine, diamorphine) can delay gastric emptying and thus slow paracetamol absorption 2

4. Overdose management: For known or suspected acetaminophen overdose within 4 hours of presentation, activated charcoal should be administered prior to starting N-acetylcysteine 1, 6

Understanding the relatively rapid onset of action of paracetamol (15-30 minutes) helps clinicians appropriately set patient expectations and make informed decisions about pain management strategies.