 This trial consists of 2 parts: a double-blinded phase and an open-label extension phase. The blinded phase only will be described in this record. Participants will be randomized in a 4:1 ratio to receive GWP42003-P or matching placebo. The hypothesis is that levels of clobazam CLB and its major metabolite N-CLB may be altered increased or decreased as a result of using GWP42003-P. 