see also:

• anti-emetics
• serotonin receptors and pharmacology

• a commonly used anti-emetic agent
• actions include:
  • 5-HT3 antagonist
    • ⇒ actions on GIT myenteric plexus:
      • ⇒ decreased bowel hypermotility & increased gastric emptying
    • ⇒ actions on CNS:
      • ⇒ anxiolytic, neuroleptic, & blocks CTZ/vomiting centre
  • 5-HT4 agonist
    • GIT neuroexcitatory ⇒ diarrhoea
    • CNS effects
    • cardiac muscle stimulation
  • CNS dopamine D2 antagonist
    • ⇒ antinausea and anti-emetic actions on the chemoreceptor trigger zone (CTZ)
    • ⇒ extrapyramidal reactions such as oculogyric crisis, dystonic reactions, akathisia such as dystonic reactions, akathisia, oculogyric crises and long term use may cause irreversible tardive dyskinesia
  • peripheral dopamine D2 antagonist (thus reduced inhibition of cholinergic smooth muscle stimulation in GIT)
    • ⇒prokinetic actions in GIT

• main adverse effects are extrapyramidal effects including:
  • dystonic reactions and oculogyric crises
    • these are usually self-limiting and rarely cause permanent damage, but they are alarming to the patient
    • generally occur within 72 hours of exposure to metoclopramide
    • highest risk tend to be younger patients with only 10% of cases occurring in those > 40yrs age
    • the higher risk in children means that it should be avoided in childhood
  • akathisia
    • defined as a movement disorder characterised by a feeling of inner restlessness and anxiety
    • symptoms ranging from mild restlessness to high levels of anxiety.
    • occurs in ~10-15% of patients receiving iv doses or infusions of 10-20mg in ED
  • exacerbation of restless legs syndrome
  • permanent, rarely reversible, tardive dyskinesia
    • this is an extremely troubling adverse effect and risk is related to:
      • older age
      • female gender
      • duration of treatment / number of doses
    • does not reverse even on withdrawal of metoclopramide
    • thus avoid long term dosing

• children - as high incidence of adverse reactions
• PH hypersensitivity or significant adverse reactions

• commonly used as a first line anti-emetic in the ED
• although often given prophylactically to combat the emetic effects of parenteral doses of opiates and opioids, there is little evidence to support this usage and exposes the patient to a significant risk of troubling akathisia unnecessarily.
• if vomiting persists and an alternative anti-emetic is indicated, then second line is usually the more expensive ondansetron

• 10mg iv, im or orally 6-8hrly prn in adults