This is a double blind cross-over study. There are two dose ranges in this study. A cross over design means that some patients will start at one dose range and others with another. About half way through the study, patients will then be switched over to the other dose range. Patients partially stabilized on lamotrigine will be randomized to either subtherapeutic (16mg) or therapeutic (200mg) dose of methylene blue. This design is necessary because methylene blue stains urine and thus it is not possible to use a traditional placebo. Double-blind means that neither the doctor nor the patient will know which strength of study drug the patient will be on at what point in the study. The study will be conducted in three centres, each recruiting 20 subjects over a two-year period. The duration of the study is 6 months.