CBD for Epilepsy: What Japanese Studies Show and Their Limits

A small Japanese survey published in 2022 reported fewer seizures among some people using CBD supplements for difficult-to-treat epilepsy. It was an encouraging observation, but it did not prove that CBD works for every epilepsy syndrome or across Asian populations. A subsequent Japanese trial of prescription cannabidiol also needs to be part of that picture.

What the Japanese Survey Actually Found

The Neurology Asia paper by Yuji Masataka and colleagues concerned CBD supplement users in Japan, not a study of Vietnamese children. Questionnaires were sent to 38 patients or families; 28 responded. Fifteen respondents reported reduced seizure frequency, including two reporting seizure resolution. Nine reported suspected adverse events, all described as mild, with none stopping CBD because of them. Median reported intake was 12 mg/kg/day.

This was a cross-sectional, self-reported survey without a randomized comparison group. Selection, nonresponse and recall could influence the findings. The small sample’s lack of a significant association between diagnosis and response does not establish equal effectiveness across seizure types. Its dose figure describes what participants used; it is not a treatment recommendation.

A Later Japanese Trial Missed Its Primary Endpoint

In an August 2024 trial update, Jazz Pharmaceuticals (JAZZ ) reported that its Japanese phase 3 study of cannabidiol oral solution did not meet its primary efficacy endpoint. The open-label, single-arm study evaluated add-on treatment for seizures associated with Lennox-Gastaut syndrome, Dravet syndrome or tuberous sclerosis complex.

The prespecified primary analysis involved 62 pediatric patients and changes in indication-associated seizure frequency over a treatment period of up to 16 weeks. The company reported numerical improvements and no new safety signals, but those observations do not turn a missed primary endpoint into a positive trial. This was a sponsor’s top-line report, and the study did not include a placebo group.

That result does not erase positive randomized trials conducted elsewhere. It does mean the earlier survey should not be presented as having settled the question for Japan or Asia as a whole.

What Does Drug-Resistant Epilepsy Mean?

Epilepsy involves a tendency to have recurrent seizures arising from abnormal brain activity. Seizures can affect awareness, movement, sensation or behavior; they do not always cause convulsions or loss of consciousness. The National Institute of Neurological Disorders and Stroke describes a range of seizure types and causes, rather than a single process involving damaged neurons.

The International League Against Epilepsy definition centers on continued seizures despite adequate use of two suitable, tolerated antiseizure medication regimens. This warrants specialist assessment. It does not mean that every treatment has failed or that CBD is a cure. Depending on the individual diagnosis, additional medicines and other specialist treatments may still help.

Where Stronger Evidence Supports Prescription CBD

Randomized trials compare CBD with placebo while participants continue their usual treatment. Their findings apply to the studied formulation, patients and seizure outcomes.

• Lennox-Gastaut syndrome: a 2018 trial involving 225 patients found median reductions in drop-seizure frequency of 41.9% with 20 mg/kg/day of cannabidiol and 37.2% with 10 mg/kg/day, compared with 17.2% with placebo. These are reductions in seizure frequency, not percentages of patients cured. Adverse events caused seven CBD recipients to stop trial medication; elevated liver enzymes also occurred.
• Dravet syndrome: a 2017 randomized trial of 120 children and young adults found that median monthly convulsive seizures fell from 12.4 to 5.9 with cannabidiol, versus 14.9 to 14.1 with placebo. Cannabidiol was added to standard treatment. This human trial provides more direct clinical evidence than an animal-model paper.

The U.S. EPIDIOLEX label covers seizures associated with Lennox-Gastaut syndrome, Dravet syndrome and tuberous sclerosis complex in patients aged one year and older. This does not approve retail CBD oils for epilepsy or establish their equivalence to the prescription medicine.

Starting Doses Are Not Maintenance Doses

Under the U.S. label, EPIDIOLEX starts at 2.5 mg/kg twice daily, totaling 5 mg/kg/day. For Lennox-Gastaut or Dravet syndrome, the usual maintenance dose is 5 mg/kg twice daily; selected patients may receive up to 10 mg/kg twice daily. For tuberous sclerosis complex, the recommended maintenance dose is 12.5 mg/kg twice daily, reached through titration.

These are prescription-label distinctions, not instructions to calculate a child’s retail CBD dose. The treating specialist must consider indication, tolerability, liver function, other medicines and the precise formulation.

Side Effects and Interactions Require Monitoring

The prescription label calls for liver tests before treatment and during follow-up. CBD can increase exposure to clobazam’s active metabolite, and concomitant valproate increases liver-enzyme risk. Dose changes require clinical supervision. Do not abruptly stop antiseizure treatment.

The FDA’s CBD information also warns about liver injury, drug interactions and increased drowsiness when CBD is combined with alcohol or other substances that slow brain activity. “Non-intoxicating” does not mean free of medical risks. Mild adverse events in a small survey cannot establish safety for everyone.

What Patients and Caregivers Can Take From the Research

Prescription CBD is an evidence-based option for certain seizure disorders, while broader claims about CBD supplements remain much less secure. Bring questions about CBD to the epilepsy team, along with the exact product and full medicine list. Ask which seizure outcome would be tracked, what monitoring is needed, and when treatment would be reconsidered. Keep following the existing seizure action plan and prescribed rescue-treatment instructions.