Significant behavioral changes after fentanyl administration occur with a delay and significantly increased fentanyl concentration. While changes in MV (moving average shown) for the subject in (a) occur shortly after the initial fentanyl exposure at 3 minutes, noticeable increases in RT to stimuli occur closer to 7 minutes in the displayed individual subject. (b) Simulating 10,000 bootstrapped samples shows an average delay of 277 seconds (95% CI: 219.5 to 332.5 seconds) in RT changes from MV changes (c). The MVI drops to 51% (95% CI: 31% to 79%) upon the onset of an increase in RT (d). Comparing the fentanyl concentration upon initial decline of MV against the concentration upon changes to RT shows a 1754-fold (95% CI: 839 to 2854 fold) increase in fentanyl concentration underlying these behavioral changes (d). *10% decrease in MV; **noticeable increase in RT.