The adherence phenomenon is now well recognized to seriously compromise drug efficacy. In this paper, we analyze the role of compliance through drug intake history as an integral part of the pharmacokinetic process. Being concerned with what is accessible in medical practice, we develop a stochastic approach to model the drug intake behavior that we combine with a conventional pharmacokinetic model in order to investigate the effect of drug intake history on the pharmacokinetic time-course. For this purpose, we explicitly formalize the plasma concentration variations for the most common administration routes. This analytical approach allows to characterize drug concentration variations directly inherited from patient compliance.