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Acute Pain | Chronic Pain | General

Injected morphine in postoperative pain

Clinical bottom line:

A 10 mg intramuscular dose of morphine is an effective analgesic. It has an NNT of 2.9 (2.6-3.6) for at least 50% pain relief over 4 to 6 hours compared with placebo in pain of moderate to severe intensity. The incidence of minor adverse effects is significantly higher with 10 mg intramuscular morphine compared with placebo.

Few trials of acceptable quality have been carried out. This is especially true for subcutaneous and intravenous studies.


Morphine is the archetypal analgesic for use in moderate or severe pain. It is the principle alkaloid of opium, and is given by many different routes. Morphine is the "gold standard" against which other injected analgesics are tested.

Systematic review

HJ McQuay, D Carroll, RA Moore. Injected morphine in postoperative pain: a quantitative systematic review. Journal of Pain and Symptom Management 1999; 17:164-74.

Inclusion criteria were full journal publication of trials of injected morphine in acute postoperative pain; single dose treatment of injected morphine and placebo (intravenous, intramuscular or subcutaneous); randomised; blinded design; moderate to severe baseline pain; adult populations.

For each trial the mean TOTPAR or SPID values for intramuscular morphine and placebo groups were converted to the percent of maximum total pain relief based on the categorical pain scales (%maxTOTPAR or %maxSPID). These values were then converted to dichotomous information on the proportion, and then the number of patients, who achieved at least 50%maxTOTPAR. A number-needed-to-treat for at least 50% pain relief and the relative benefit of the treatment were then calculated. Adverse effects frequency data were used to calculate numbers-needed-to-harm and relative risk.

Findings

Morphine was given by intramuscular injection in all trials except one intravenous trial. Sufficient data for meta-analysis was available for 10 mg intramuscular morphine trials only. The NNT for 10 mg intramuscular morphine was 2.9 (2.6 to 3.6).

Figure: At least 50% pain relief with intramuscular morphine 10 mg compared with placebo

Adverse effects

Minor adverse effects occurred in 34% of patients given intramuscular morphine compared with 23% with placebo. This was significantly higher than placebo, with an NNH of 9.1 (5.6 to 27). There was no difference between morphine and placebo with major adverse effects (drug-related study withdrawal).

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