Figure 5.

Codeine, but not morphine, levels correlate between plasma and brain; brain morphine levels reflect codeine analgesia. Plasma and brain codeine levels correlated significantly (a) at 15 min, suggesting that codeine equilibrated quickly between the plasma and brain compartments. There was no correlation between plasma and brain morphine levels following codeine administration (b). Analgesia correlated with brain morphine levels (c) but not with plasma morphine (d) at 15 min after codeine administration (n=26). Pearson correlations were used.