Please use this link to access this publication. Abstract Aim To evaluate the long-term effectiveness of cannabidiol (CBD)-enriched oil for the treatment of refractory epilepsy and to assess the development of tolerance to its anti-seizure effect. Methods A prospective study of 92 consecutive patients (age 1–37 years, mean-11.8 years) with treatment resistant epilepsy who were treated with cannabis oil extract (CBD/tetrahydrocannabinol [THC] ratio of 20:1). Mean monthly seizure frequency was reported by the patients/their parents during monthly clinic visits. Tolerance was defined as either the need to increase the dose by ≥30% due to reduced treatment efficacy or as an increase of ≥30% in...