A small but critical trial revealed that much of CBD's somnolence in epilepsy is caused by a drug interaction with clobazam, not by CBD itself. Also confirmed the CBD-valproate liver interaction and dose-proportional pharmacokinetics.
Neurologists prescribing Epidiolex, parents of Dravet patients managing medication combinations, anyone wanting to understand CBD's drug interactions.
CBD raises clobazam metabolite levels — explaining much of the drowsiness
The Backstory
The Dravet NEJM trial tested one dose: 20 mg/kg/day. It worked. But clinical medicine needs to know more than whether a drug works at one dose. It needs to know: what happens at lower doses? Does the safety profile change? How does the drug interact with the medications these patients are already taking? What's the minimum effective dose and the maximum tolerable one?
This small, carefully designed dose-ranging trial answered those questions for the youngest Dravet patients — children aged 4 to 10.
The Study
How They Did It
GWPCARE2: CBD Dose-Ranging in Young Dravet Patients
Design
Randomized, double-blind, dose-ranging safety trial. Three CBD dose levels plus placebo. Classified as providing Class I evidence.
The primary goal was safety characterization, not seizure efficacy
Population
34 children aged 4-10 with Dravet syndrome. Randomized 4:1 across three CBD doses and placebo.
10 patients at 5 mg/kg/day, 8 at 10 mg/kg/day, 9 at 20 mg/kg/day, 7 placebo
Duration
Treatment period followed by safety monitoring. 32 of 34 patients (94%) completed the study.
High completion rate — even in this young, severely affected population
Key measurements
Pharmacokinetics (drug levels in blood), drug interactions with concurrent anti-epileptic drugs, liver function, and adverse event rates across dose levels.
Particular attention to clobazam interaction and hepatotoxicity
Devinsky et al. (2018), Neurology 90:e1204-e1211; PMCID:PMC5890607
The Key Findings
This trial's most important contributions weren't about seizures — they were about the pharmacology of CBD in combination with other epilepsy drugs.
Process
What the Dose-Ranging Trial Revealed
Dose-proportional pharmacokinetics
CBD exposure (AUC) increased proportionally with dose — no unexpected accumulation or nonlinear kinetics. Double the dose, double the blood levels. This makes dosing predictable.
Clobazam interaction identified
CBD significantly increased levels of N-desmethylclobazam (N-CLB), the active metabolite of clobazam — one of the most commonly prescribed drugs for Dravet syndrome. This interaction was NOT seen in patients also taking stiripentol.
Hepatotoxicity signal with valproate
Six patients taking CBD plus valproate developed elevated liver transaminases. None met formal criteria for drug-induced liver injury. All recovered. No elevations occurred in patients not taking valproate.
Dose-dependent adverse events
Pyrexia, somnolence, decreased appetite, sedation, vomiting, and ataxia occurred more frequently with CBD than placebo. Higher doses produced more adverse events.
Devinsky et al. (2018), Neurology 90:e1204-e1211
The Clobazam Story
Myth vs. Reality
CBD's side effects in epilepsy trials are caused by CBD itself.
Many of the adverse effects reported in CBD epilepsy trials — particularly somnolence and sedation — are at least partially caused by drug interactions, not by CBD directly. CBD inhibits CYP2C19, which raises levels of N-desmethylclobazam (the active metabolite of clobazam, a commonly co-prescribed drug). CBD also interacts with valproate through CYP pathways, causing liver enzyme elevations. Understanding these interactions is essential for safe prescribing — and it means CBD's direct side effect profile may be milder than the trial data suggests.
The Evidence
Devinsky et al. (2018): CBD increased N-CLB levels in a dose-proportional manner except in patients on stiripentol. Six patients on CBD+valproate had elevated transaminases; none on CBD without valproate did.
Devinsky et al. (2018), Neurology 90:e1204-e1211
Small Trial, Big Impact on Practice
With only 34 patients, this study wasn't powered to measure seizure reduction with statistical confidence. Its value was different: it characterized the pharmacology that makes safe prescribing possible. Every neurologist who writes an Epidiolex prescription and adjusts the concurrent clobazam dose is relying on data from this trial.
Related Research
The Dosing and Safety Evidence
This trial filled in the pharmacological details that the efficacy RCTs didn't address.
Trial of Cannabidiol for Drug-Resistant Seizures in the Dravet Syndrome
Devinsky et al. (2017)
The efficacy trial (20 mg/kg/day). This dose-ranging study tested 5, 10, and 20 mg/kg/day.
Add-on CBD for tuberous sclerosis complex
Thiele et al. (2021)
The TSC trial confirmed the dose-ceiling finding: 25 mg/kg was as effective as 50 mg/kg with fewer side effects.
Long-term CBD safety/efficacy in LGS/DS
Laux et al. (2019)
Long-term data showing the safety profile is stable through 144 weeks — no new signals beyond what this dose-ranging study identified.
What is the right dose of CBD for epilepsy?
Based on the full Epidiolex trial program, 20 mg/kg/day is the established starting therapeutic dose for Dravet and Lennox-Gastaut syndromes. The TSC trial showed 25 mg/kg/day was as effective as 50 mg/kg/day with fewer side effects. This dose-ranging study showed that lower doses (5-10 mg/kg/day) are tolerated but may be less effective. Dose titration should be individualized by a neurologist, with particular attention to concurrent medications (clobazam, valproate) that interact with CBD.
Why does CBD cause drowsiness in some epilepsy patients?
Much of the somnolence seen in CBD epilepsy trials is likely caused by a drug interaction, not CBD directly. CBD inhibits the CYP2C19 enzyme, which causes the active metabolite of clobazam (N-desmethylclobazam) to accumulate to higher-than-expected levels. Since clobazam is a benzodiazepine (inherently sedating), elevated levels produce drowsiness. Reducing the clobazam dose when adding CBD often resolves the somnolence.
What the researchers found
CBD pharmacokinetics are dose-proportional in young Dravet patients. The critical drug interaction: CBD increases N-desmethylclobazam (active clobazam metabolite) levels via CYP2C19 inhibition — explaining much of the somnolence in CBD trials. CBD+valproate causes liver enzyme elevations (6 patients affected, all recovered). Adverse events are dose-dependent.
Why it matters
Characterized the pharmacology that makes safe Epidiolex prescribing possible. The clobazam interaction discovery changed clinical practice — neurologists now routinely reduce clobazam doses when adding CBD. The valproate-liver interaction confirmed the need for hepatic monitoring.
The numbers in context
34 patients (10 at 5 mg/kg, 8 at 10 mg/kg, 9 at 20 mg/kg, 7 placebo). 32/34 completed (94%). CBD exposure dose-proportional (AUC). 6 CBD+valproate patients had elevated transaminases. N-CLB levels increased with CBD except on stiripentol.
How the study worked
Randomized, double-blind, dose-ranging safety trial across multiple sites. Children 4-10 with Dravet syndrome. Three CBD doses (5, 10, 20 mg/kg/day) plus placebo, randomized 4:1. Pharmacokinetic sampling, drug interaction assessment, liver function monitoring.
Who was studied
Children aged 4-10 with Dravet syndrome on stable anti-epileptic drug regimens
What this study cannot tell us
Very small sample size (34 total, 7 placebo). Not powered for seizure efficacy outcomes. Short treatment duration. Age range 4-10 may not generalize to older patients.
How to read the evidence
Classified as Class I evidence by Neurology for the safety question. Small sample (34 patients) limits power for efficacy conclusions. Well-designed dose-ranging study with pharmacokinetic measurements.
When this study was published
Published 2018. Findings are now incorporated into Epidiolex prescribing information and standard clinical practice.
The bigger picture
The efficacy trials proved CBD works. This trial explained how to use it safely alongside the drugs these patients are already taking. Every neurologist who adjusts a clobazam dose when prescribing Epidiolex is relying on this data.
Questions still open
- Would lower CBD doses (5-10 mg/kg) provide meaningful seizure reduction with fewer drug interactions? Can the clobazam interaction be leveraged therapeutically (lower CBD dose + lower clobazam dose for equivalent effect)?
Common questions
Read the original research
Randomized, dose-ranging safety trial of cannabidiol in Dravet syndrome
Neurology
Neurology — the official journal of the American Academy of Neurology, the most widely read and highly cited peer-reviewed neurology journal.
Citation
Devinsky O, Patel AD, Thiele EA, Wong MH, Appleton R, Harden CL, Greenwood S, Morrison G, Sommerville K. (2018). Randomized, dose-ranging safety trial of cannabidiol in Dravet syndrome. Neurology. https://doi.org/10.1212/WNL.0000000000005254