GW Pharmaceuticals PLC said it plans to charge about $32,500 per patient annually in the U.S. for its new treatment for rare forms of epilepsy, the first prescription drug derived from the marijuana plant.

The U.S. Food and Drug Administration approved Epidiolex, also known as cannabidiol, in June to reduce seizures associated with forms of epilepsy known as Lennox-Gastaut syndrome and Dravet syndrome, in patients 2 years of age and older.

GW...