It is a well known fact that therapeutic doses of morphine can cause known significant respiratory depression, especially in an opioid naïve patient. Other factors like, the route of administration, concurrent medication, respiratory reserve and age of the patient etc. are some of the factors that have a bearing on the dose that can cause respiratory depression. It has been reported that the risk of patients developing respiratory depression with postoperative morphine is of the magnitude of ~ 1-2%, irrespective of route it and moreover, can be severe enough to cause death. The risk can persist for a long time after a single dose, especially for an extended release preparation. This warrants a strict vigil and observation to guard for respiratory depression, especially necessary in opioid naïve patients and those with other risk factors mentioned above, in whom the starting dose has to be low.

Having said all that, it is difficult to exactly predict a safe dose that will not cause respiratory depression in any patient, since the therapeutic window is narrow and any dose of morphine that causes analgesia will cause some respiratory depression also. Patients at a higher risk of fatal respiratory depression need to be monitored very closely.Kindly refer to the following reference for further clarification. At least you will get therapeutic window in which good analgesia is balanced with acceptable amount of respiratory depression.

Coetzee JF. S Afr J Anaesthesiol Analg 2010;16(2).

Neeta, thanks for your answer.

I have experience of an IV dose as low as 5mg causing respiratory depression requiring ICU admission and intubation as the junior medic did not check/realise that the patient had chronic renal failure.