Figure 5. Isobologram for dose providing 50% maximum possible effect (MPE) with 95% confidence intervals (CIs) (arrows) at the time of peak effect (15 min) in clonidine-morphine combinations. The x,y coordinates represent equieffective pairs of doses of clonidine and morphine, respectively. Isoboles C and M represent 50% MPE dose of clonidine and morphine used alone, respectively. Isobole A represents the actual experimental 50% MPE doses (0.6 micro gram for clonidine and 0.3 micro gram for morphine). Isobole B represents the theoretical additive 50% MPE doses of clonidine and morphine (2.6 micro gram for clonidine, 1.3 micro gram for morphine). If their interaction was purely additive, it would have required 2.6 micro gram clonidine plus 1.3 micro gram morphine to produce the 50% MPE. As displayed, isobole A fell below isobole B; in addition, 95% CIs of the two isoboles did not overlap each other. This finding indicates significant synergism between clonidine and morphine (P < 0.05).