In a landmark placebo-controlled trial of 224 patients with tuberous sclerosis complex, cannabidiol at 25 mg/kg/day reduced seizures by 48.6% compared to 26.5% for placebo, with the lower dose showing a better safety profile.
Epileptologists, TSC patients and families, CBD prescribers
CBD reduced TSC seizures by 48.6% vs 26.5% for placebo (30.1% reduction from placebo)
The Backstory
The Dravet trial proved CBD worked for one catastrophic epilepsy. The Lennox-Gastaut trial proved it worked for a second. But both conditions primarily involve generalized seizures — electrical storms that sweep across the entire brain. The question remained: would CBD work for focal seizures, where the electrical chaos starts in one specific region?
Tuberous sclerosis complex was the test case. TSC is a genetic disease that grows non-cancerous tumors throughout the body — brain, kidneys, heart, skin, lungs. In the brain, these growths called cortical tubers create focal seizure hotspots. About 80-90% of TSC patients develop epilepsy, often before their first birthday. The seizures are focal first, though they frequently generalize. And they resist medication.
If CBD worked here — in a structurally different disease with primarily focal seizures — it would prove that cannabidiol's anticonvulsant mechanism was truly broad-spectrum.
The Disease
The Trial
How They Did It
GWPCARE6: CBD for Tuberous Sclerosis Complex
Design
Double-blind, placebo-controlled, three-arm randomized clinical trial — the most rigorous design yet for CBD in epilepsy, with two active dose groups.
46 sites across 6 countries: USA, UK, Australia, Netherlands, Poland, Spain. NCT02544763
Population
224 patients aged 1-65 years with a clinical diagnosis of TSC and medication-resistant epilepsy. Required at least 8 seizures during a 4-week baseline period. Must have failed at least 1 anti-epileptic drug.
Median age 11.4 years. 75 randomized to CBD 25 mg/kg/day, 73 to CBD 50 mg/kg/day, 76 to placebo
Intervention
Oral cannabidiol (Epidiolex) at two dose levels — 25 mg/kg/day (CBD25) or 50 mg/kg/day (CBD50) — versus matched placebo for 16 weeks.
The two-dose design answered a question the previous trials couldn't: is more CBD better?
Primary endpoint
Percentage change from baseline in TSC-associated seizure frequency during the 16-week treatment period.
Included focal seizures — the dominant seizure type in TSC
Thiele et al. (2021), JAMA Neurol 78:285-292; NCT02544763
The Results
CBD 25 mg/kg/day (n=75)
- Seizure reduction: 48.6%
- Reduction from placebo: 30.1% (p < 0.001)
- Diarrhea: 31%
- Somnolence: 13%
- Discontinued for AEs: 11%
- Elevated liver enzymes: included in 18.9% overall CBD rate
Effective with better safety profile
CBD 50 mg/kg/day (n=73)
- Seizure reduction: 47.5%
- Reduction from placebo: 28.5% (p = 0.002)
- Diarrhea: 56%
- Somnolence: 26%
- Discontinued for AEs: 14%
- Elevated liver enzymes: included in 18.9% overall CBD rate
No more effective than lower dose, substantially more side effects
Placebo (n=76)
- Seizure reduction: 26.5%
- Diarrhea: 25%
- Somnolence: 9%
- Discontinued for AEs: 3%
- Elevated liver enzymes: 0%
Substantial placebo response — consistent with other epilepsy trials
Thiele et al. (2021), JAMA Neurol 78:285-292
48.6% vs. 47.5%
seizure reduction at the 25 mg/kg/day and 50 mg/kg/day doses — virtually identical efficacy. But the adverse event profile was dramatically different: diarrhea jumped from 31% to 56%, somnolence from 13% to 26%, and discontinuations from 11% to 14% at the higher dose. More CBD was not better — it was just more toxic.
This is the first CBD epilepsy trial to demonstrate a clear dose ceiling. The Dravet and LGS trials tested only 20 mg/kg/day. This trial showed that doubling the dose adds side effects without adding benefit.
Thiele et al. (2021), JAMA Neurol 78:285-292
Why This Trial Changed the Conversation
Process
What the TSC Trial Added to the CBD Evidence
CBD works for focal seizures
TSC seizures are primarily focal in origin. The Dravet and LGS trials focused on generalized seizures. This trial extends CBD's proven efficacy to focal seizure types — a mechanistically important expansion.
CBD works in structural epilepsy
TSC has a visible structural cause (cortical tubers). Dravet is a channelopathy. LGS has mixed etiologies. CBD now works across structural, genetic, and heterogeneous epilepsy causes.
More is not better
25 mg/kg/day was as effective as 50 mg/kg/day with substantially fewer side effects. This has direct clinical implications: start lower, don't escalate unless needed.
Third FDA indication
Led to Epidiolex's approval for TSC-associated seizures in 2020 — expanding the drug from two rare epilepsies to three.
Thiele et al. (2021), JAMA Neurol 78:285-292
The Liver Signal
Myth vs. Reality
CBD is completely safe because it's natural and non-psychoactive.
At therapeutic epilepsy doses, CBD causes elevated liver transaminase levels in 18.9% of patients — nearly 1 in 5. Zero patients on placebo had this finding. The elevation is dose-dependent and worsened by concurrent valproate use. While most cases are reversible with dose reduction, it requires regular blood monitoring. At therapeutic doses, CBD is a drug with real hepatotoxicity risk, not a harmless supplement. This is why Epidiolex is prescription-only with mandated liver function monitoring.
The Evidence
Thiele et al. (2021): 18.9% of CBD patients had elevated liver transaminases vs 0% placebo. Dose-dependent: higher rates at 50 mg/kg/day. Interaction with valproate documented across all three Epidiolex trials (Dravet, LGS, TSC).
Thiele et al. (2021), JAMA Neurol 78:285-292
Related Research
Three Epilepsies, One Drug
This trial completed the trifecta: Dravet, Lennox-Gastaut, and now tuberous sclerosis complex. Each added evidence that CBD is a broad-spectrum anticonvulsant.
Cannabidiol in patients with treatment-resistant epilepsy
Devinsky et al. (2016)
The Dravet open-label + NEJM RCT — first indication. Generalized seizures, sodium channelopathy.
CBD for Lennox-Gastaut syndrome (GWPCARE4)
Thiele et al. (2018)
Second indication. Drop seizures, heterogeneous etiologies. Confirmed CBD generalizes beyond Dravet.
Chronic administration of cannabidiol to healthy volunteers and epileptic patients
Cunha, Carlini, Mechoulam et al. (1980)
The first CBD epilepsy trial — ignored for 40 years. Temporal lobe (focal) epilepsy — the same seizure origin that TSC produces.
A tale of two cannabinoids
Russo & Guy (2006)
CBD as an independent therapeutic agent — a claim now supported by three FDA-approved indications.
What is tuberous sclerosis complex?
TSC is a genetic disease (affecting ~1 in 6,000 people) caused by mutations in the TSC1 or TSC2 gene, which normally suppress cell growth via the mTOR pathway. When these genes are dysfunctional, benign tumors grow in the brain, kidneys, heart, lungs, and skin. In the brain, growths called cortical tubers create seizure hotspots. About 80-90% of TSC patients develop epilepsy, often in the first year of life. Seizures are typically focal (starting in one brain region) and frequently resist medication.
Is the higher dose of CBD better for seizures?
No. This trial specifically showed that 25 mg/kg/day was as effective as 50 mg/kg/day (48.6% vs 47.5% seizure reduction) but with substantially fewer side effects — diarrhea dropped from 56% to 31%, somnolence from 26% to 13%. The clinical recommendation is to use the lower effective dose. More CBD is not better for seizure control and comes with significantly more adverse effects and hepatotoxicity risk.
Does this mean CBD works for all types of epilepsy?
CBD has now been proven effective in three distinct epilepsy types: Dravet syndrome (genetic channelopathy, generalized seizures), Lennox-Gastaut syndrome (heterogeneous causes, drop seizures), and tuberous sclerosis complex (structural, focal seizures). This breadth suggests a broad-spectrum anticonvulsant mechanism. However, CBD has not been studied in all epilepsy types, and efficacy in these three severe conditions does not guarantee it will work for milder or different forms. Clinical trials for other indications are ongoing.
What the researchers found
Both CBD doses significantly reduced TSC-associated seizures versus placebo: 48.6% reduction for 25 mg/kg/day (30.1% reduction from placebo, p<0.001) and 47.5% for 50 mg/kg/day (28.5% from placebo, p=0.002), with no additional benefit from the higher dose but more adverse effects.
Why it matters
This is the pivotal trial demonstrating CBD efficacy for seizures in tuberous sclerosis complex, a condition characterized primarily by focal seizures, expanding CBD's proven epilepsy indications beyond Lennox-Gastaut and Dravet syndromes.
The numbers in context
224 patients randomized (75 CBD25, 73 CBD50, 76 placebo); 201 completed treatment; median age 11.4 years; seizure reduction: CBD25 48.6%, CBD50 47.5%, placebo 26.5%; diarrhea: 25% placebo, 31% CBD25, 56% CBD50; elevated liver enzymes in 18.9% on CBD vs 0% placebo.
How the study worked
Double-blind, placebo-controlled RCT (GWPCARE6) of 224 patients aged 1-65 with TSC and medication-resistant epilepsy across 46 sites in 6 countries, comparing oral CBD at 25 mg/kg/day and 50 mg/kg/day to placebo over 16 weeks.
What this study cannot tell us
Elevated liver enzymes in 18.9% of CBD patients (none on placebo); side effects increased substantially with higher dose; 16-week duration may not capture long-term efficacy or tolerance; concomitant antiepileptic medications could interact.
How to read the evidence
Large multi-site double-blind placebo-controlled RCT published in JAMA Neurology, providing strong evidence.
When this study was published
Trial enrolled 2016-2018, published in 2021.
The bigger picture
The finding that the lower CBD dose was as effective as the higher dose with fewer side effects has practical implications for dosing strategies and could reduce treatment costs and improve tolerability.
Questions still open
- Does seizure reduction persist long-term, or does tolerance develop? Would even lower CBD doses maintain efficacy with fewer side effects?
Common questions
Does CBD help seizures in tuberous sclerosis?
Is the higher dose better?
Read the original research
Add-on Cannabidiol Treatment for Drug-Resistant Seizures in Tuberous Sclerosis Complex: A Placebo-Controlled Randomized Clinical Trial.
JAMA neurology, 78(3), 285-292
Citation
Thiele, Elizabeth A; Bebin, E Martina; Bhathal, Hari; Jansen, Floor E; Kotulska, Katarzyna; Lawson, John A; O'Callaghan, Finbar J; Wong, Michael; Sahebkar, Farhad; Checketts, Daniel; Knappertz, Volker. (2021). Add-on Cannabidiol Treatment for Drug-Resistant Seizures in Tuberous Sclerosis Complex: A Placebo-Controlled Randomized Clinical Trial.. JAMA neurology, 78(3), 285-292. https://doi.org/10.1001/jamaneurol.2020.4607
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