When are clinical trials of a given agent vs. placebo no longer appropriate or feasible?

In several situations, placebo-controlled trials may be difficult to conduct. This article discusses the reasons for such difficulty--principally, a belief that the effect of a therapy has already been established or the existence of another therapy already proved effective. In the latter case, an active control equivalence trial may be considered, but this design has important deficiencies in many situations, notably, the lack of assurance of assay sensitivity.