Paracetamol
Mechanism of Action: The exact mechanism is poorly understood, with various sites of action identified and proposed [1].
- Prostaglandin inhibition: peroxide-dependent COX inhibition. Peroxide concentrations are low in the brain where paracetamol has a strong effect, with less effect at peripheral sites of inflammation where peroxide levels are high.
- Activation of descending serotonergic pathways
- Action on the endocannabinoid system via the paracetamol metabolite AM404 [2], which inhibits the reuptake of anandamide
- Other metabolites act on the TRPA1 receptor [3]
The relative analgesic effect of these different mechanisms is unknown.
Presentation: Tablet, oral suspension, effervescent tablet (high Na+ content), capsule (not available in OUH), rectal suppository (restricted), solution for intravenous infusion (available as 1000mg in 100ml or 500mg in 50ml: flush the giving set to achieve full dose).
Suggested dose: 1g four times a day for adults over 50kg with normal hepatic function
Reduce the dose if underweight:
- Less than 40kg, give 500mg four times a day
- 40-49.9kg, give 1g three times a day
- 50kg and above, give 1g four times a day
or if hepatic compromise (e.g., 1g three times a day)
Absorption:
- Oral: bioavailability of 63-89%, mainly through the small bowel. Onset of analgesia: 40-60min
- Rectal: bioavailability very variable: 24-98%. Onset of analgesia 40-60min
Protein binding: low
Lipid solubility: high
Metabolism: Hepatic, mainly by glucuronidation and sulphation to non-toxic conjugates
A small percentage is oxidized by the cytochrome P450 system to the highly toxic metabolite N-acetyl-p-benzo-quinone imine (NAPQ1). This is normally conjugated with glutathione and renally excreted, however if there is insufficient glutathione (as in paracetamol overdose or glutathione deficiency) NAPQ1 causes hepatic necrosis.
NAPQ1 may also disrupt the vitamin K cycle so can potentiate the effect of warfarin [4]. Patients on warfarin taking regular doses of paracetamol should have more frequent INR testing, while those who present with unexplained INR variability should be asked about paracetamol ingestion as a possible contributing factor.
Toxicity: Paracetamol overdose can cause fatal acute hepatotoxicity, and is typically pictured as a single large ingestion with early presentation to ED. Acetylcysteine is used to treat overdose: BNF poisoning emergency treatment. OUH Acetylcysteine MIL
Patients with hepatotoxicity following a staggered overdose (where repeated supratherapeutic doses are taken, often accidentally) are more likely to have delayed presentation, be encephalopathic on admission, require renal replacement therapy or mechanical ventilation, and have higher mortality [5]. Patients are often unaware that over the counter (OTC) preparations such as Lemsip contain paracetamol.
The following contains a list of the contents of some OTC medicines: OTC medicines
Elimination: Mainly renal excretion of the metabolites and parent drug. There is slower elimination of metabolites if the eGFR is less than 30ml/min, so the dose interval should be a minimum of 6 hours.
Half-life: 1.5-2.5 hours
Further prescribing information (side effects, contraindications, interactions):