According to Merriam-Webster dictionary, the word "Equipoise" means a state of equilibrium. In clinical trial, the concept of 'clinical equipoise' means that there is genuine uncertainty over whether a treatment will be beneficial. In other words, in randomized controlled clinical trials, there should be substantial uncertainty or there is no clear evidence that one treatment arm is particularlly better or worse. Clinical equipoise provides the ethical basis for medical research involving patients assigned to different treatment arms of a clinical trial - it is unethical to assign a subject to an inferior arm if the lack of equipoise exists and if there is substantial evidence that one treatment is better or worse than another treatment.
In the real word, when we plan a randomized, controlled clinical trial, 'lack of equipoise' may often exist. This is especially true in late stage clinical trials. In late stage clinical trials, there typically be some evidences about the benefit and treatment effect of the experimental drug from early phase I or phase II clinical trials. Our sample size calculation is based on the assumed treatment effect of the experimental drug.
We recently published a paper in the Journal of Neurology "Challenges of clinical trial design when there is lack of clinical equipoise: use of a response-conditional crossover design" where we discussed a situation of 'lack of equipoise' and the use of a response-conditional crossover design to ease the concern about the lack of equipoise in clinical trial design. Several small trials had suggested that IVIg is beneficial in treating the disease CIDP - lack of equipoise. However, in the absence of an approved treatment for this indication, gaining regulatory approval for the use of IVIg in this indication required the conduct of large-scale, placebo-controlled confirmatory trials. Using the response-conditional crossover design, we eased the concern about subjects exposed to the 'perceived' inferior arm (Placebo in this case). The results indicated that we minimized subject's exposure to the inferior treatment arm.
Interpretation of 'clinical equipoise' may be different among clinicians, investigators, patients, clinical trial sponsors, and regulatory agencies. Evidences of treatment effects from small-scale clinical trials may be thought as real evidence for clinician and patients, but not for regulatory agencies.
When we bring the overall benefit/risk into the picture for assessing the 'clinical equipoise', it may be difficult to determine whether or not a clinical equipoise exist or not. A new treatment may have been demonstrated beneficial in efficacy, but with great uncertainty in safety.