Cannabidiol has shown efficacy in randomized clinical trials for drug-resistant epilepsy in specific syndromes that predominantly affect children. However, high-level evidence for the efficacy and safety of cannabidiol in the most common form of drug-resistant epilepsy in adults, focal epilepsy, is lacking.

Question: What is the efficacy, safety, and tolerability of transdermally administered cannabidiol in adults with drug-resistant focal epilepsy?

Findings: In a randomized, double-blind, placebo-controlled, multicenter clinical trial of 188 patients, no difference was found in seizure frequency at week 12 of the double-blind period among the placebo, 195-mg cannabidiol, and 390-mg cannabidiol treatments. The open-label extension demonstrated the long-term safety, tolerability, and acceptability of transdermal cannabidiol delivery, with a seizure reduction of at least 50% in more than half of the patients by month 6 of the trial.

Meaning: Although cannabidiol did not perform significantly better than placebo in this trial, it was well tolerated and safe; future studies to assess the effect of higher doses may be warranted.