Analgesic efficacy of paracetamol and its combination with codeine and caffeine in surgical pain--a meta-analysis

The objective of this study was to quantify the analgesic efficacy of paracetamol and its combination with codeine or caffeine through a systematic overview and meta-analysis of relevant randomized controlled trials (RCTs). Systematic retrieval of relevant clinical trials was carried out using computerized searches, historical searches and communication with manufacturers. The results of RCTs were pooled to estimate (i) the difference in percentage improvement of total pain relief (TOTPAR%) and the sum of pain intensity difference (SPID%); (ii) the proportions of patients obtaining moderate to excellent pain relief relative to placebo (ResRR) and (iii) the ratio of patients requiring analgesic re-medication (RemRR). Head-to-head comparisons were also undertaken for paracetamol versus its combination with codeine or caffeine. A total of 80 RCT reports describing 103 placebo comparisons and 26 head-to-head comparisons were identified. The total pain relief score in the single dose studies increased by 38 percentage points for paracetamol and by 24 points for placebo. The difference (d) in TOTPAR% between the two was highly significant (d = 14, 95% CI: 12, 16). For the difference in SPID%, d = 12, 95% CI: 11, 13. Patients were more than twice as likely to obtain moderate to excellent pain relief on paracetamol than on placebo (ResRR = 2.39, 95% CI: 1.89, 3.02), and less likely to require re-medication (RemRR = 0.78, 95% CI: 0.69, 0.88). There was no significant (P > 0.05) dose-response relationship. The analgesic efficacy of paracetamol 600 mg was enhanced with the addition of codeine 60 mg (using TOTPAR% as outcome) in both indirect and head-to-head comparisons. SPID%, but not ResRR and RemRR, data supported this conclusion. Much weaker effects were observed with the caffeine combination. Adverse effects were mild. Surprisingly, drowsiness was seen more often with paracetamol and paracetamol-codeine combinations than with placebo. The relative risks (95% CI) were 1.83 (1.29, 2.59) and 2.39 (1.58, 3.57), respectively. In conclusion paracetamol is an effective analgesic for post-surgical pain. Caffeine adds little to the analgesic effect of paracetamol. However, there is some evidence that codeine 60 mg adds to the analgesic effects of paracetamol 600 mg, using pain relief or pain intensity scores as outcomes, but this is not necessarily translated into an increase in number of patients who obtain moderate to excellent pain relief.