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The respective roles of controlled clinical trials and observational studies (cohort or case-control studies) in evaluating the efficacy, safety and usefulness of a drug were analysed. A randomised, controlled, double-blind study is the best method of estimating the efficacy of a treatment. It provides the least biased and most robust estimate of the causal relationship. In certain situations and on the basis of certain criteria, observational studies can have a proof-of-efficacy value. Randomised, controlled, pre-and postmarketing authorisation (MA) clinical studies identify the rarer adverse effects and compare them with those resulting from the reference treatment. Before the MA, the pooled safety data from different controlled trials can provide an estimation of relatively frequent adverse events and subjects at risk. However, an observational study is the most appropriate method of evaluating the safety of a drug in the currently used conditions. By definition, a drug influences the health of a population if it directly or indirectly improves its health. A drug would have a major role in public health if it reduced mortality or morbidity related to a particular disease or if it improved the quality of life of patients with this disease. Prior to marketing a product, modelling is the approach of choice to quantify the expected effect. Pragmatic, postmarketing trials and observational studies are the reference methods used to define the population affected, the efficacy and safety of the drug in a real situation and its usefulness for public health. In conclusion, randomised clinical trials remain the reference approach for evaluating efficacy, while observational studies have a confirmatory value. Observational studies are the most appropriate way of evaluating safety in the currently used conditions, as the clinical trial has limited indications. In the interests of public health, modelling is the pre-marketing approach of choice, while pragmatic trials and observational studies are the postmarketing reference approaches.

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