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Acute Pain | Chronic Pain | General

Aspirin in postoperative pain

Clinical bottom line

A single oral dose of aspirin is effective in the relief of postoperative pain (doses 600 mg to 1200 mg). A dose of 600/650 mg had an NNT of 4.4 (4.0 to 4.9) for at least 50% pain relief over 4 to 6 hours compared with placebo in pain of moderate to severe intensity. This is as effective as 1000 mg of paracetamol, but is associated with increased adverse effects (gastric irritation and nausea).

Aspirin (Acetylsalicylic Acid) is an important analgesic. It is widely available, and prescriptions of 300 mg tablets total approximately 3/4 million annually in England alone (1996).

Systematic review

JE Edwards, AD Oldman, LA Smith, PJ Wiffen, D Carroll, HJ McQuay, RA Moore. Oral aspirin in postoperative pain: a quantitative systematic review. Pain 81 (1999) 289-97.

JE Edwards, A Oldman, L Smith, SL Collins, D Carroll, P Wiffen, HJ McQuay, RA Moore. Single dose aspirin in acute pain. The Cochrane Library, Update Software, Oxford 2000 (updated with no additional results 2002).

Inclusion criteria were full journal publication; randomised placebo controlled trials of aspirin; postoperative oral administration; adult patients; group size at least 10; double blind; standard pain outcomes; baseline pain moderate to severe.

Mean TOTPAR and SPID values for each trial were converted to %maxTOTPAR and %maxSPID, and then the proportion of patients achieving at least 50%maxTOTPAR were calculated. This information was used to calculate NNT and relative . Adverse effects frequency data were used to calculate numbers-needed-to-harm and relative risk.

Findings

Aspirin was significantly superior to placebo with single oral doses of 600/650 mg, 1000 mg and 1200 mg. A dose of 600/650 mg had an NNT of 4.4 (4.0 to 4.9). Aspirin 500 mg was not effective for pain relief. There was insufficient data to establish the efficacy of 300/325 mg and 900 mg doses.

Figure: Comparisons of various doses of aspirin with placebo in postoperative pain.

Table: Summary of relative benefit and number-needed-to-treat for trials of aspirin compared with placebo

Aspirin dose (mg) Number of comparisons Patients with at least 50% pain relief with aspirin Patients with at least 50% pain relief with placebo Relative benefit (95%CI) NNT (95%CI)
500 3 45/135 32/115 1.2 (0.8 to 1.8) 18 (5.9 to -17)
600/650 68 960/2499 404/2562 2.0 (1.8 to 2.2) 4.4 (4.0 to 4.9)
1000 7 153/357 64/359 2.2 (1.4 to 3.4) 4.0 (3.2 to 5.4)
1200 5 85/140 27/139 3.3 (1.8 to 6.3) 2.4 (1.9 to 3.2)

Adverse effects

A meta-analysis was carried out for aspirin 600/650 mg versus placebo. All adverse effects analysed together generated a relative risk of 1.2 (1.03to 1.4), and an NNH of 44 (23 to 345). At this dose, there was a significantly higher incidence of drowsiness, NNH 28 (19 to 52) and gastric irritation, NNH 38 (22 to 174), but not of nausea, vomiting, dizziness or

Table: Adverse effects: aspirin 600/650 mg compared with placebo

Dose   Number of trials

Patients with adverse effects with aspirin

Patients with adverse effects with placebo

Relative risk (95%CI) NNH (95%CI)
All doses Total adverse effects 60 313/2619 261/2660 1.3 (0.9 to 1.5) not calculated
Aspirin 600/650 mg Total adverse effects 53 257/1976 229/2088 1.2 (1.03 to 1.4) 44 (23 to 345)
  Dizziness 30 41/1429 27/1557 1.6 (0.9 to 2.6) not calculated
  Drowsiness 33 103/1542 56/1672 1.9 (1.4 to 2.5) 28 (19 to 52)
  Gastric irritation 11 20/546 6/562 2.5 (1.2 to 5.1) 38 (22 to 174)
  Headache 29 34/1237 56/1363 0.7 (0.4 to 1.02) not calculated
  Nausea 34 54/1563 68/1683 0.8 (0.6 to 1.2) not calculated
  Vomiting 21 12/835 18/927 0.7 (0.4 to 1.6) not calculated

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