The drug effect-time profile may not follow the drug conc in plasma or the drug conc in the peripheral compartment.
This indicates that the drug at the site of action is not in rapid equilibrium with the drug in plasma, or the drug conc-effect relationship changes with time. Also the drug may produces its effect by indirect mechanism.
In this case when the drug conc is plotted versus the drug effect the resulting profile will look like a hysteresis loop.
This means that at a given plasma drug conc, the drug effect will be different depending on whether the drug conc is increasing or decreasing.
This hysteresis can be clockwise or counterclockwise (as in the graph below).
