[From research to practice: clinical phases for drug development].

Clinical trials become very relevant in the development of new drugs when their pharmacokinetics, pharmacodynamics, efficiency, safety and possible adverse effects are being assessed. So new drugs are available for their daily use in patients, a model has been proposed for more than four decades; This model consists in the realization of sequential research studies, which were called clinical phases I, II, III and IV, which begin once the drugs' effects have been verified in cellular and animal models (preclinical phase). In this article, the general characteristics of each of the clinical phases are synthesized but, apart from that, the modifications that have been done over the years are described with the purpose of making new drugs available in a quicker way.