How does cannabis reduce seizures?
The short answer: it's CBD, not THC, doing most of the work. Purified CBD is the only cannabis compound with FDA approval for epilepsy, sold as Epidiolex for Dravet syndrome, Lennox-Gastaut syndrome, and tuberous sclerosis complex. CBD doesn't hit the same receptor THC does. It quiets overactive brain signaling through a handful of other targets, and it changes how your body clears some seizure meds. That second part matters more than most people realize.
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It's also not a cure and not a fit for every seizure type. In trials, some patients saw big drops in seizure frequency and others saw almost none. This is an add-on treatment, not a replacement for what your neurologist prescribed.
What CBD does in the brain
THC binds CB1 receptors hard. CBD barely touches them. Instead, CBD works on several fronts at once:
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- Adenosine: CBD blocks a transporter that pulls adenosine out of the space between neurons. More adenosine around means less excitability.
- GPR55: CBD blocks this receptor, which normally revs up neuronal firing.
- TRPV1 and 5-HT1A: CBD interacts with both, which may calm the circuits that help start a seizure.
- GABA-A and glycine receptors: These are the brain's brakes. CBD nudges them toward more inhibition.
- T-type calcium channels: CBD blocks a channel type linked to absence seizures and thalamic rhythms.
- FAAH: CBD slows the enzyme that breaks down anandamide, so your own endocannabinoids stick around longer.
No single one of these explains the effect. The likely answer is that several of them stack.
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THC is a different story
THC acts on CB1 receptors directly, and the seizure data there is mixed. Some animal studies show THC raising seizure threshold, others show it lowering it at high doses. There isn't enough human evidence to lean on THC for seizure control. People use THC-dominant products for sleep, appetite, or pain alongside epilepsy, and that's a separate conversation from seizure reduction. If seizure control is the goal, CBD is the compound with the research behind it.
The clobazam interaction nobody mentions
This one is worth understanding. CBD blocks the CYP2C19 enzyme. Clobazam, a common epilepsy drug, gets broken down by that enzyme into norclobazam, which is also active. Add CBD and norclobazam levels can climb several times higher. Some researchers think part of CBD's seizure benefit in trials came from this boost rather than from CBD alone. It also means more sedation and more drug interactions. CBD affects CYP3A4 too, which touches a long list of medications. If you're on clobazam, valproate, warfarin, or anything with a narrow margin, your neurologist needs to know before you add CBD.
What the trials found
The GW Pharmaceuticals trials that led to FDA approval tested 10 and 20 mg/kg per day of purified CBD against placebo. In Dravet syndrome, the CBD group saw a median drop in convulsive seizures around 39%, versus about 13% on placebo. In Lennox-Gastaut, it was roughly 37% to 42% versus about 17%. In tuberous sclerosis complex, about 48% versus 27%. Those are medians, which means half did better and half did worse. A minority became seizure-free or close to it. A minority saw no change.
That dose is far above what a typical bottle of CBD oil delivers. A 40-pound child at 10 mg/kg is taking 180 mg a day. Most drugstore bottles are 10 to 25 mg per dropper.
If you're looking at CBD products
Over-the-counter CBD is not held to the same standard as a prescription drug. Independent testing has found a large share of products mislabeled, sometimes with more CBD than claimed, sometimes with less, and sometimes with THC that wasn't on the label. What I look for:
- A batch-specific certificate of analysis from an accredited third-party lab, not a generic one.
- Clear labeling of total CBD per bottle and per serving.
- THC content stated, which matters if you're subject to drug testing or want to avoid psychoactive effects.
- A brand that answers questions about sourcing and extraction.
Hemp-derived CBD with no more than 0.3% THC is federally legal in the US, but state rules vary and some states cap THC tighter. Epidiolex itself is a prescription drug, Schedule V at the federal level, and you get it through a pharmacy, not a dispensary.
Side effects to expect
Sleepiness, diarrhea, lower appetite, and fatigue show up often. The one that needs monitoring is liver enzyme elevation. CBD can raise ALT levels, and the risk climbs when valproate is in the mix. Doctors running CBD for epilepsy typically check liver panels before starting and again in the first months. Drowsiness tends to be worse when clobazam is on board.
What to do next
Bring it up with your neurologist rather than going around them. Track seizures with dates, times, and descriptions, since that's the only way to know whether something is working. Don't stop or lower a prescribed anticonvulsant on your own. Ask directly about interactions with your current meds. And give it time. Seizure frequency changes slowly, and most trials measured response over 12 to 14 weeks.
Cannabis is not a cure for epilepsy, and for a lot of people it won't move the needle. For some, it cuts seizures enough to change daily life. The difference between those two outcomes comes down to the right compound, the right dose, and medical supervision.