[Response variability in pharmacotherapy]

The response obtained to drug administration can be modified by several factors which, from a synthetic point of view, can be classified into four basic groups: physical, chemical, biological and therapeutical. The first two can be verified and controlled by means of in-vitro test and, they must be known and solved before de final formulation is obtained. The other two factors, biological and therapeutical ones, introduce variables whose analysis concerns to the prescriptor. After this introduction, the variability in drug response is analyzed and discussed paying special attention to the pharmacokinetic and pharmacodynamic variability, analyzing the sources of such variability: environmental factors, factors depending of age, physical and genetical constitution, pathological states or drug interactions.