Figure 2

Isobolograms for the administration of the combination of methadone with morphine (2 A), methadone with fentanyl (2 B), tramadol and morphine (2 C), fentanyl and morphine (2 D),tramadol with methadone (2 E) and tramadol with fentanyl (2 F), in the writhing test of mice. Theoretical ED₅₀ value with 95% CL (●). Experimental ED₅₀ value with 95% CL (○).