In this paper we analyse the dynamics of an inhibitor I which can either bind to a receptor R or to a plasma protein P. Assuming typical association and dissociation rates, we find that after an initial dose of inhibitor, there are three time scales: a short one, measured in fractions of seconds, in which the inhibitor concentration and the plasma-protein complex jump to quasi-stationary values, a medium one, measured in seconds in which the receptor complex rises to an equilibrium value and a large one, measured in hours in which the inhibitor-receptor complex slowly drops down to zero. We show that the average receptor occupancy, the pharmacologically relevant quantity, taken over, say, 24h reaches a maximal value for a specific value of the plasma-protein binding constant. Potentially, understanding and exploiting this optimum could be of great interest to those involved in drug discovery and development.