Additive analgesic effect of codeine and paracetamol can be detected in strong, but not moderate, pain after Caesarean section - Baseline pain-intensity is a determinant of assay-sensitivity in a postoperative analgesic trial

Background: A randomized, double-blind, placebo-controlled single oral dose study was done in order to examine whether codeine has an additive analgesic effect to that of paracetamol for moderate and strong postoperative pain after abdominal surgery. The maximum recommended single dose of paracetamol 1000 mg (Paracet(R)) was compared with a combination of a submaximal dose of paracetamol 800 mg plus codeine 60 mg (Paralgin forte(R)) and placebo for pain relief after Caesarean section in 125 patients. Methods: Visual analogue pain intensity score (VAS 0-100 mm) and categorical pain relief score were recorded for 6 hours after the study drug intake. The main efficacy variables analyzed were: pain intensity difference and summed pain intensity differences during the first 3 and 6 h after study drug intake, total pain relief during the first 3 and 6 h, global evaluation score at the end of the observation period, and time to rescue analgesic. Results: Because of protocol violations, 17 patients were excluded from the analysis of effects. Among the 108 patients included in the analysis of analgesic effect, 49 patients had moderate baseline pain (VAS between 40 and 60 mm on a 100 mm scale), and 59 patients had strong baseline pain (VAS more than 60 mm). In patients with strong baseline pain, statistically highly significant differences were documented in efficacy variables between the active drugs and placebo and between the two active drugs. However, in patients with moderate baseline pain, no differences were found between the study drugs in any of the analgesic efficacy variables. Conclusion: This study thus confirms that codeine has additive analgesic effect to paracetamol in pain after surgery. Our results show the importance of initial pain intensity in postoperative assessment of analgesic drugs. Assay-sensitivity and test power are increased by selecting patients with sufficiently high initial pain intensity and by comparing groups of patients with identical surgery and similar demographic variables.