Fewer clinical trials focus specifically on progressive-relapsing MS (PRMS) than any other course of MS for two important reasons:
- PRMS may be more difficult to diagnosis or to distinguish from other disease courses, at least initially.
- PRMS is the least common disease course, which means that it is difficult to recruit adequate numbers of people to participate.
Depending on the specific inclusion criteria used in a given trial, people with PRMS may or may not be invited to participate. When they are included, it tends to be in trials for symptomatic management medications — see, for example, the trials for fampridine-SR and cannabis extract.