Converting from oral to parenteral morphine

Teaching Exercise 1: Converting from oral to parenteral morphine

Mr. John Smith, a terminally ill lung cancer patient is a resident in your nursing home care unit. He has chronic cancer pain that is well controlled by Morphine Sustained Action, 90mg every 12 hours. Today, during rounds, Mr. Smith complains of painful swallowing and is unable to take his Morphine Sustained Action pills. An exam reveals that he has severe oral candidiasis (thrush). You start him on nystatin swish and swallow and also decide to temporarily change him to a Morphine infusion. Calculate the hourly basal dose of morphine infusion.

Step 1 Calculate current total dose oral Morphine over 24 hours

2 x 90 mg = 180 mg

Step 2 Convert to equivalent dose of Parenteral morphine by dividing total 24 hour oral dose by 3

(Morphine oral to Parenteral dose ration is 1:3)

180 mg divided by 3 = 60 mg

Step 3 Calculate hourly dose of Parenteral morphine by dividing the total 24 hour dose by 24.

60 mg / 24 = 2.4 mg

Therefore the hourly basal rate of Morphine sulfate infusion should be 2.54 milligrams.

Step 4 Calculate break through dosage:

Rule of thumb: Total amount of break through dosage available in 24 hours = total amount of basal drug given in 24 hours.

In this case, as patient is getting a total basal dose of 60 mg, s/he should have available total break through dose of:

60 mg / 24 = 2.4 mg

Step 5 Write break through pain medication orders:

Morphine injections for break through pain can be safely given every 15 minutes (peak effect of IV Morphine is about 5 minutes).

In this case, patient’s available breakthrough should be 60mg/ 24 hours.

2.5 mg / hr = 0.625 mg / 15 minutes

Thus the final order should read: Start Morphine Sulfate sub-cutaneous infusion at a basal rate of 2.5mg per hour with a break through dose of 0.625mg every fifteen minutes as needed. See Using Opioid Infusions to Palliate Pain.
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