Wednesday, 3 November 2010

MRCP revision battle 41.5: Opioid and benzo conversions

Occasionally in real-life/MRCP exam you may need to convert between different drugs in the same family, so this revision battle briefly covers 2 popular families: the opioids and the benzos.


First, as an aside: remember opiate refers to a substance derived from opium (from the poppy) while opioids are synthetic/semi-synthetic versions of opiates.

  • PO codeine to PO morphine: divide by ten
  • PO tramadol to PO morphine: divide by five
  • PO morphine to PO oxycodone: divide by two
  • PO morphine to S/C morphine: divide by two
  • PO morphine to S/C diamorphine: divide by three
  • PO morphine to S/C oxycodone: divide by three
  • morphine to fentyl: divide by 100

Other misc notes:
  • codeine relies on conversion to an active form by the P450 enzyme group.  Not all patients are able to do this (roughly 10% of the population struggle), and other drugs (such as SSRIs) may interfere with the effectiveness of conversion so the same amount of codeine may have a very variable analgesic effect on different people.
  • doses of all opioids should be reduced in renal failure except fentanyl
  • if asked to prescribe a breakthrough dose this is usually 1/6th of the total 24hr dose.

Benzos (and derivative)

Rough conversions are:

Chlordiazepoxide 15mg = Temazepam 10mg = Diazepam 5mg = Lorazepam 0.5mg

Decrease by 1/8th every 2 weeks.

Now for a little pyrexia of unknown origin...