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Introduction to opioids

  • Junior doctors will often be asked prescribe opiods
  • Ehether it is starting a patient on it for the first time, or writing up an admitting drug chart it is helpful to understand a bit more about opioids and what adverse effects to look out for.
  • ‘Opiate’ vs ‘opiod’
    • Opiates are drugs derived from opium. ‘Opiods’ used to refer synthetic opiates only (drugs created to mimic opium, however different chemically). However, ‘opiod’ is now used for the entire family of opiates including natural, synthetic and semi-synthetic.


Pharmacology of opioids

  • Types of opioids
    • Natural opioid (opiates): codeine, morphine
    • Semi-synthetic opioids: oxycodone, hydrocodone, diamorphine, buprenorphine
    • Synthetic opioids: methadone, fentanyl


  • Weak Opioids vs Strong Opioids
    • Weak: codeine, dihydrocodeine
    • Strong: tramadol, morphine, oxycodone, fentanyl, buprenorphine, diamorphine, methadone


  • Morphine Metabolism
    • Opioids are predominantly metabolised by the liver where they undergo first pass metabolism
    • The two main metabolites are M3G and M6G (morphine-3 or 6-glucononide)
    • Morphine metabolites are then excreted by the kidneys
      • Therefore use with caution in patients with renal impairment: the metabolite will not be excreted as fast risking gradual build up and overdose over time


Prescribing opioids

  • WHO principles of cancer pain management
    • By the mouth: preferred first line route
    • By the clock: persistent pain responds better to regular analgesia than PRN
    • By the ladder: see WHO pain ladder
    • For the individual: see pain pages
    • Attention to detail: review response regularly
  • Morphine is the first line strong opioid of choice in palliative care
  • Consider what route is best to manage the pain
    • Oral route is first line but if the patient is vomiting or unable to take medications orally, subcutaneous injection (s/c) may be preferred. Remember s/c route is twice the potency of oral
  • Start with immediate release preparation (IR)
    • Once the person’s opioid requirements are established you can switch them to a modified release preparation or patch


Cautions with morphine

  • A caution is not a contraindication: you can still prescribe morphine in these situations but be aware of how its effect might be different
  • Caution in patients with renal and hepatic impairment
    • Not excreted so quickly – more risk of overdose
  • Caution in the elderly
    • Tend to need lower doses, more inclined to become delirious with opioids
  • Caution in inflammatory bowel disorders
    • May increase complication rate including toxic megacolon
  • Caution in obstructive airways disease
    • Can lead to respiratory depression, risking CO2 retention
  • Caution in epilepsy
    • May lower the seizure threshold – uncommon


Side effects of morphine

  • Once started on an opioid remember to regularly review for common side effects: (the * indicates toxicity: immediately review +/- discussion with specialist palliative care team)
  • Constipation and nausea
  • Dry mouth
  • Confusion*
  • Drowsiness *
  • Myoclonus *
  • Hallucinations *
  • Pinpoint pupils * (can be present in chronic opioid use)
  • Respiratory depression if severe (RR<8) *


Peak plasma concentrations morphine

  • Immediate release (oral) – levels usually peak within the first hour, effect lasts for 4 hours
  • Modified release (oral) – levels usually peak at 2-6 hours and effect lasts for 12 hours.


Example starting prescription of morphine

  • Ensure that you read local and national prescribing guidelines and refer to relevant formularies.
  • NB: PRN doses should be approximately 1/6 of the total dose received in a 24 hour period
  • Moderate pain:
    • Morphine Sulphate IR (oramorph) 2.5mg 4-6 hourly regularly (i.e. QDS or six times per day)
      • AND
    • Morphine Sulphate IR (oramorph) 2.5mg PRN max 2 hourly
  • Severe pain:
    • Morphine Sulphate IR (oramorph) 5mg 4-6 hourly regularly (i.e. QDS or six times per day)
      • AND
    • Morphine Sulphate IR (oramorph) 2.5-5mg PRN max 2 hourly
  • NB. if starting a regular opioid remember to start a laxative (e.g. Movicol I sachet BD orally or sodium docusate 200g BD) and antiemetic (e.g. metoclopramide 10mg po/iv TDS as required or cyclizine 50mg po/iv TDS as required).


Dose calculations in opioids

  • When calculating opioid doses, switching to an alternative opioid or switching route doses are usually referred to by their strength relative to oral morphine.
  • It always helps to refer to a dose calculation chart when prescribing a new opioid to give you an idea of how much the patient is receiving in a 24 hour period.


Useful opioid dose calculations

  • Codeine
    • Codeine 60mg = 6mg oral morphine (divide by 10)
  • Tramadol
    • Tramadol 100mg = 10mg oral morphine (divide by 10)
  • Morphine
    • Morphine sulphate 10mg iv = morphine sulphate 20mg PO (multiply by 2)
    • Morphine sulphate 10mg sc = morphine sulphate 20mg PO (multiply by 2)
  • Oxycodone
    • Oxycodone IR 5mg PO = morphine sulphate IR 10mg PO (multiply by 2)
    • Oxycodone 5mg sc = morphine sulphate IR 20mg PO (multiply by 4)
  • Patches
    • Patches have varying equivalent doses and a dose equivalent range is often cited.
    • Buprenophine patch normally changed every 7 days
    • Buprenorphine 10 micg/hour = 20mg morphine sulphate PO in 24 hours
    • Buprenorphine 20 micg/hour = 45 mg morphine sulphate PO in 24 hours
    • Fentanyl patch normally changed every 3 days (72 hours)
    • Fentanyl 12micg/hour = 45 mg morphine sulphate PO in 24 hours
    • Fentanyl 25 micg/hour = 90 mg morphine sulphate PO in 24 hours.


Click here for how to manage opioid toxicity

Perfect revision for medical students, finals, OSCEs and MRCP PACES

Click here for how to choose and prescribe analgesia other than opioids

Click here to download free teaching notes on opioids