Experimental research proceeds by hypotheses formulated on the basis of previous or new knowledge and then tested. If they are accepted, they serve as the basis for further hypotheses, and if they are rejected newhypotheses can be developed. In other words, when we are at the frontiers of knowledge the path is forged by "trial and error". When a trial shows a hypothesis is wrong, this is a step toward making fewer errors.This process also applies to drug development. There is nomagic formula at present to predict-at the pre-clinical level-the therapeutic value of a drug for peoplewith a disease. However, pre-clinical studies are needed in order to formulate hypotheses that justify clinical trials. Without these preliminary studies in vitro and in vivo in selected animal species it would be unethical to test still unproven chemicals in humans. (C) 2016 European Federation of Internal Medicine. Published by Elsevier B.V. All rights reserved.