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Rate‐suppressing effects of morphine before (top), during (top) and after (bottom) daily treatment of either morphine alone (left) or morphine in combination with 2‐BFI (right). Ordinate: responding rate as a percentage of control responding rate. Abscissa: dose of morphine (mg·kg−1); n = 9 per group. Linear regression was used to estimate the ED50 (±95% CL) values of morphine across the different weeks.
