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.
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
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.
