As such they gain licences for adjunctive therapy in patients with uncontrolled focal epilepsies from short placebo controlled
trials determining the ‘responder rate’ for a seizure reduction of 50% or more. These refractory patients do not represent
the population with epilepsy and so there are limitations in applying the evidence from these trials in the real world. Postlicensing studies examine longer-term effects, such as the visual field constriction later found with vigabatrin.