Histamine H1 R antagonists - antiemetic

• What is the indication of histamine H1-R antagonists

  Prophylaxis/ treatment of nausea + vomiting

  Particularly motion sickness/vertigo

  Allergies
• Name 3 histamine H1- R antagonists

  THW ZINES …

  Cyclazine

  Promethazine

  Cinnarizine
• Histamine H1- r antagonist MOA

  Inhibits vestibular and vomiting centre communication by blocking histamine H1 and muscarinic ACh receptors

  CYCLIZINE - blocks both receptors

  Used in multiple emission e.g drug induced and motion
• Common side effects of of H1- r antagonists

  Drowsiness - cyclizine = least

  Dry throat/mouth - anticholinergic effect - good for copious mucosal secretions

  Post IV = transient tachycardia (palpitations)
• when is the use of histamine h1-r antagonists cautioned

  • Hepatic encephalopathy - due to hr-r antagonists sedating effects
  • Prostatic enlargement - pt will be susceptible to anticholinergic effect e.g urinary retention !
• What are anticholinergics and there side effect

  • block the action of ACh
  • Drowsiness
  • Rapid HR
  • Urinary retention (unable to empty bladder)
  • Flush skin (unable to sweat)
  • Constipation
  • Dry eyes/ mouth
• what do histamine H1 r antagonists reactive with

  Sedative drugs (benzodiazepines, opioids) - greater sedation

  Ipratopium/ Tiotropoum -pronounced anticholinergic effect
• what is the typical dosing of cyclizine

  50 mg . 8-hourly PRN

  Route depends on clinical situ for active emesis - IV>IM = Less pain

  IV - slowly over 2 mins
• Considerations for stoping histamine H1 -R antagonists

  Should be PRN

  If taken regularly, periodic interruption to see if still needed is advised
• what is another drug available OTC for motion sickness

  Hyoscine Hydrobromide - antimuscarinic

  Transdermal patch behind ear a few hours before departure

  One 1.5mg patch= up to 72 hours (1mg is absorbed)