Design issues for drug epidemiology.

Despite the difficulties involved in designing drug epidemiology studies, these studies are invaluable for investigating the unexpected adverse effects of drugs. The aim of this paper is to discuss various aspects of study design, particularly those issues that are not easily found in either textbooks or review papers. We have also compared and contrasted drug epidemiology with the randomized controlled trial (RCT) wherever possible. Drug epidemiology is especially useful in the many situations where the RCT is not suitable, or even possible. The study base has to be defined before the appropriate cohort of subjects is assembled. If all of the cases are identified, then a referent sample of controls may be assembled by random sampling of the study base. If all of the cases cannot be assembled, a hypothetical secondary base may need to be created. Preferably, only new-users of the drug should be included, and the risk-ratio will be different for acute users and chronic users. Studies will usually only be possible when researching the unintended effects of drugs. It is difficult to study efficacy because of confounding by indication. In occasional circumstances it may be possible to study efficacy (examples are given). Discussion of the dangers of designing with generalisability in mind is provided. Additionally, the similarities in study design between drug epidemiology and the RCT are discussed in detail, as well as the design-characteristics that cannot be shared between the two methods.