Fig. 6. Model prediction of the effect site morphine-6-glucuronide (M6G; thick line ) concentration versus  analgesic effect—pain tolerance—and comparison with the morphine concentration-effect relation (thin line ) from Sarton et al.  4using pooled male and female data. The effect axis is relative to baseline: a value of 1.25 indicates 25% increase in current relative to baseline. At C²⁵, the potency ratio morphine to M6G is 1:12; at C⁵⁰, the ratio is approximately 1:22.