Onset of Opioid Analgesia

When would you expect that a patient would experience the onset of analgesic effect after taking a dose of Morphine Sulphate Immediate Release tablets (MSIR)?

a.      5 – 10 minutes
b.   20 – 30 minutes
c.    45 – 60 minutes
d.   75 – 90 minutes

Answer: b.  

The onset of activity of all short-acting opioids is approximately 20-30 minutes.  Patients who report no analgesic benefit after 1 hour should not be told to “give it a while longer”, but instead, be dose escalated.  Another common error is the idea that opioids accumulate over time-patients are instructed to “give it a few days” before a change in dosage.  In fact, for patients with normal renal and hepatic function, continued dosing does not accumulate and thus there is no rationale to continue using an ineffective dose.

Want to learn more? Read Fast Fact #18 Opioid Doing Intervals