Lennox-Gastaut syndrome (LGS) is a rare and severe form of epilepsy. It starts in childhood and usually continues into adult life. Most people with LGS still have seizures despite trying many medicines.
A study published in the European Journal of Neurology in 2026 looked at 79 adults with LGS at King’s College Hospital in London. Each had been prescribed a licensed cannabidiol (CBD) medicine alongside an epilepsy medicine called clobazam.
This article is general information about a research study. It is not medical advice and is not a guide to treatment or dosing. The CBD medicine in this study is licensed in the UK for seizures in LGS when used with clobazam. It is prescription-only and is started by specialist epilepsy doctors.
Key Takeaways:
- The study. Doctors at King’s College Hospital looked back at the records of 79 adults with LGS who started CBD between January 2019 and April 2025. Half were followed for 41 months (about 3.5 years) or longer.
- Who took part. Most had lived with epilepsy for decades. Before starting CBD, 65 of 79 (82.3%) were taking 3 or more epilepsy medicines.
- Seizures. After 6 months, drop seizures had fallen by more than half in 39 of 78 people (50.0%). At the last check, this figure was 48 of 78 (61.5%). No one became seizure-free.
- Daily life. On average, seizure-free days rose from about 6 to about 12 a month. Carers reported less use of rescue medicines and fewer emergency hospital admissions.
- Side effects. Drowsiness (33 people) and diarrhoea (24 people) were the most common. CBD changed the levels of some other epilepsy medicines, and 51 people needed lower doses of at least one other medicine. Eight people (10.1%) stopped CBD.
- What it cannot show. There was no comparison group, and results came from medical records and carers’ diaries. The study cannot show how much of the change was due to CBD.
What is Lennox-Gastaut Syndrome?
Lennox-Gastaut syndrome (LGS) is a rare type of epilepsy that begins before the age of 18, most often before the age of 8. Doctors recognise it by three main features:
- Several types of seizures. At least one type must be tonic seizures, where the body suddenly stiffens.
- Learning and behavioural difficulties. These may not be obvious when seizures first start.
- A typical pattern on an EEG. An EEG (electroencephalogram) is a test that records the brain’s electrical activity.
Many people with LGS have drop seizures. These are seizures that cause a sudden stiffening or loss of muscle tone in the whole body, trunk or head. They can lead to falls, injuries or slumping in a chair.
LGS has many possible causes. These include genetic changes, differences in how the brain developed, and brain injury around the time of birth. In many people, no cause is found. Seizure types and EEG patterns can change over time. Seizure freedom is rare.
How is Lennox-Gastaut syndrome usually treated in the UK?
There is no cure for LGS. Treatment aims to reduce how often seizures happen and how severe they are. The National Institute for Health and Care Excellence (NICE) epilepsy guideline (NG217) says everyone with LGS should have a neurologist with expertise in epilepsy involved in their care.
NICE sets out treatment in steps:
- First choice. Sodium valproate. Strict safety rules apply to valproate, especially for anyone who could become pregnant. Since January 2024, people under 55 can only start valproate if two specialists agree there is no other suitable option (Epilepsy Action).
- Second choice. Lamotrigine, either on its own or added to valproate.
- Further add-on options. CBD with clobazam, clobazam alone, fenfluramine, rufinamide or topiramate.
If seizures continue, specialists may consider a ketogenic diet. This is a medically supervised high-fat, low-carbohydrate diet. Other options can include a vagus nerve stimulator (VNS), a small implanted device that sends regular electrical signals to a nerve in the neck. Some people are offered surgery.
For both CBD and fenfluramine, NICE says drop seizures should be checked every 6 months. Treatment should be stopped if drop seizures have not fallen by at least 30% compared with the 6 months before starting (TA615, TA1050).
What is CBD, and why are researchers studying it in adults with LGS?
Cannabidiol (CBD) is one of the main compounds found in the cannabis plant. Exactly how CBD affects seizures is not fully understood. It acts on several targets in the brain that help control how easily nerve cells fire.
The medicine in this study is a highly purified CBD oral solution. In the UK, it is licensed for seizures linked to LGS and Dravet syndrome (both with clobazam) and tuberous sclerosis complex, in people aged 2 and over. It is not the same as CBD products sold in shops.
Large clinical trials of this medicine mostly included children and teenagers. Adults with LGS are often different. They have usually had epilepsy for many years and take several medicines at once. This makes drug interactions more likely. The authors note that there is little real-world information about CBD in adults with LGS.
What did the study do?
This was a retrospective audit. This means the team looked back at existing medical records. They did not give treatment as part of a trial.
The study took place in the adult epilepsy service at King’s College Hospital, London. It included every adult treated with CBD for LGS between January 2019 and April 2025. To be prescribed CBD, each person had to:
- have a diagnosis of LGS, based on their age at onset, seizure types and EEG results
- have ongoing seizures despite standard treatment
- meet the NICE criteria for CBD
What treatment did people receive?
NICE requires CBD to be used with clobazam. Of the 79 people, 41 were already taking clobazam. Another 37 started it, usually at least 2 weeks before CBD. One person took a similar medicine instead because they could not tolerate clobazam. People who had struggled with clobazam in the past were given small doses.
The CBD dose was increased gradually. Doctors adjusted it for each person, based on seizures and side effects. The team often increased doses more slowly than the product information suggests. This was to allow for side effects and interactions with other medicines.
People had regular blood tests to check their liver, kidneys and blood counts. Where needed, the team also measured blood levels of other epilepsy medicines.
What did the researchers measure?
Carers kept seizure diaries using a standard form. The team compared the period before CBD with 6 months after starting it. They also looked at the most recent clinic review, called the “last follow-up”. The last follow-up was taken from before any new epilepsy medicine was added.
The team recorded:
- how often each type of seizure happened, including drop seizures
- the number of seizure-free days each month
- use of rescue medicines (emergency medicines given during a long or repeated seizure)
- hospital admissions for seizures or seizure-related injuries
- changes in sleep, behaviour and thinking, as reported by carers
- side effects, blood test results and changes to other medicines
Who took part?
The 79 adults (40 men and 39 women) had a median age of 29, with ages ranging from 18 to 62.
Their history shows how hard their epilepsy had been to control:
- they had lived with epilepsy for a median of 26 years
- 66 of 79 (83.5%) had their first seizures before the age of 8
- 72 of 79 (91.1%) had a moderate or severe learning disability
- 65 of 79 (82.3%) were taking 3 or more epilepsy medicines, and had already tried a median of 6 others
- 47 of 79 (59.5%) had a working VNS device
- 7 had had brain surgery for epilepsy, and 25 had tried a ketogenic diet in the past
The cause of LGS was genetic in 21 people, a difference in brain development in 10, and a brain injury in 11. In 37 people, the cause was unknown.
The median age at starting CBD was 27. Eight people had started it as teenagers under children’s services.
What did the study find?
Did seizures change?
Yes. On average, seizures became less frequent after starting CBD. The table shows how many people had their seizures fall by more than half.
| Seizure outcome | After 6 months | At last follow-up |
| Drop seizures fell by more than half | 39 of 78 (50.0%) | 48 of 78 (61.5%) |
| Drop seizures fell by more than 90% | 7 of 78 (9.0%) | 8 of 78 (10.3%) |
| All seizures fell by more than half | 47 of 79 (59.5%) | 53 of 79 (67.1%) |
| All seizures fell by more than 90% | 5 of 79 (6.3%) | 8 of 79 (10.1%) |
These figures count everyone who started CBD, including people who later stopped it. People who stopped are counted as not having improved.
Results were similar for some other seizure types. Myoclonic seizures (sudden muscle jerks) fell by more than half in 11 of 23 people (47.8%). Absence seizures (brief “switching off”) fell by more than half in 19 of 39 people (48.7%) at 6 months and 22 of 39 (56.4%) at last follow-up.
No one became completely seizure-free. A small number of people had more of a particular type of seizure than before, even when their seizures fell overall.
Did the number of seizure-free days change?
Yes. The average number of seizure-free days each month roughly doubled.
| Time point | Average seizure-free days per month |
| Before CBD | 5.7 |
| After 6 months (75 people) | 11.1 |
| At last follow-up (62 people) | 12.5 |
These are averages. Some people had many more seizure-free days than others.
Did use of rescue medicines and hospital care change?
Carers reported less use of rescue medicines for many people. At 6 months, use had fallen for 36 of 75 people (48.0%) and risen for 5. At last follow-up, it had fallen for 31 of 64 people (48.4%) and risen for 3.
Carers also reported fewer emergency hospital admissions for seizures or seizure-related injuries in the first 6 months. This was based on information for 62 people.
Did carers notice other changes?
Carers of 48 out of 67 people (71.6%) described improvements in thinking at the last follow-up. Examples included better attention, communication and engagement. Some carers also described better sleep (19 people), behaviour (15 people), mobility or balance.
These changes were not measured with formal tests. They came from clinic letters and carers’ comments. They were not clearly linked to whether a person’s seizures improved. The study cannot show whether CBD caused them.
Did anything make a difference to who responded?
The team looked at whether the cause of LGS, having a working VNS, or the dose of clobazam or CBD made a difference. None showed a clear difference. Some groups were small, so these results are uncertain.
What side effects were reported?
Side effects were common. The most frequent were:
- Drowsiness, in 33 people. This was often linked to CBD raising the levels of other medicines.
- Diarrhoea, in 24 people. This was more common than in earlier clinical trials. It often improved when the dose was lowered, but it limited the highest dose that 17 people could take.
- Abnormal liver blood tests, in 12 people. This was more common in people also taking valproate (10 of 39) than in those who were not (2 of 40). Results improved after the valproate dose was lowered. No one stopped CBD because of liver results.
- Worse behaviour, in 11 people. This was often due to drug interactions and improved when other medicines were reduced.
- More seizures, in 6 people.
- Other stomach upset (5 people), tiredness (4), weight loss (4), reduced appetite (3) and poor balance (3).
Less common effects included a low platelet count, which affects blood clotting, in 2 people.
Did CBD affect other medicines?
Yes. CBD can change the amount of some other medicines in the blood. In this study, important interactions were seen with clobazam, valproate, lamotrigine, phenytoin, rufinamide and everolimus.
For example, one person taking a high dose of clobazam became very sleepy, with slurred speech and slower thinking, after starting CBD. These problems improved when the clobazam dose was gradually reduced.
The team expected these interactions and planned for them. Overall:
- 51 of 79 people (64.6%) had the dose of at least one other epilepsy medicine reduced
- 14 of 79 people (17.7%) were able to stop at least one other epilepsy medicine
Important: Do not stop, start or change any epilepsy medicine without talking to your neurology team. Stopping epilepsy medicines suddenly can cause severe seizures.
How many people stopped CBD?
By 6 months, 4 people (5.1%) had stopped CBD. By the last follow-up, 8 people (10.1%) had stopped. The main reasons were:
- sleepiness or lack of energy (4 people)
- changes in behaviour or mood (3 people)
- diarrhoea or vomiting (3 people)
- not enough benefit (3 people)
Some people had more than one reason. One person stopped because their seizures increased.
Sadly, 4 people died during the study. The authors report that these deaths were not related to CBD. Of the 75 people still living at the end of the study, 67 (89.3%) were still taking CBD.
What are the limitations of this study?
The authors point out several important limits:
- One hospital. Everyone was treated at a single specialist centre in London. Results may differ elsewhere.
- Looking back at records. The study used existing records, not a planned trial. Some information was missing for some people.
- No comparison group. No one received a placebo (a dummy treatment with no active ingredient). Without one, it is not possible to tell how much of the change was due to CBD.
- Carer reporting. Seizure counts came from carers’ diaries. These can be affected by memory, and seizure types can be confused.
- Small groups. There were too few people in some groups to compare, for example, by the cause of LGS.
- Informal measures. Changes in thinking and behaviour were not measured with formal tests.
Other medicines were also being adjusted during the study. Seizures in LGS go up and down naturally too. Both of these may affect the results. The lead author’s research post was funded by the company that makes the CBD medicine. The company had no role in the study but reviewed the paper before publication.
How does this study fit with other research?
The CBD medicine was licensed for LGS after two large randomised controlled trials. In these trials, people were given CBD or a placebo by chance, without knowing which they had.
According to the authors, 36% to 44% of people taking CBD had their drop seizures fall by at least half, compared with 14% to 24% of those taking placebo (Devinsky et al., 2018 and Thiele et al., 2018). Most people in these trials were children and teenagers.
The authors say their findings are in line with these trials and with other real-world studies. This study adds longer-term information in adults. It includes how long people stayed on treatment and how other medicines were managed.
For more on LGS research, see our article on how cannabinoids could affect EEG results in Lennox-Gastaut syndrome.
What does this study mean, and what does it not mean?
In this group of adults with long-standing LGS, many had fewer seizures after starting CBD with clobazam. Seizure-free days increased, and carers reported less need for rescue medicines and emergency care. Most people stayed on treatment for several years. No one became seizure-free.
Side effects were common, especially drowsiness and diarrhoea. Many people needed changes to their other medicines. The authors stress the need to plan for drug interactions when starting CBD.
The study does not show that CBD will help any one person. Because there was no comparison group, the changes cannot be put down to CBD with confidence.
Frequently asked questions
Can CBD be prescribed for epilepsy in the UK?
Yes, in some cases. A licensed CBD medicine can be prescribed for seizures linked to Lennox-Gastaut syndrome, Dravet syndrome and tuberous sclerosis complex. It is started by specialist epilepsy doctors and is available on the NHS for people who meet the NICE criteria. Other cannabis-based medicines are unlicensed in the UK. A specialist doctor may consider them when other treatments have not worked well enough.
Is medical cannabis prescribed for epilepsy?
Yes. Medical cannabis was legalised in the UK in November 2018 and can now be prescribed for a range of conditions where other therapies have failed – including for treatment-resistant epilepsy.
What did this study find?
The study looked at 79 adults with LGS who took CBD alongside clobazam. After 6 months, drop seizures had fallen by more than half in 39 of 78 people (50.0%). At last follow-up, this was 48 of 78 (61.5%). No one became seizure-free. There was no comparison group, so the study cannot show how much of the change was due to CBD.
What side effects were reported?
The most common were drowsiness (33 people), diarrhoea (24 people), abnormal liver blood tests (12 people) and worse behaviour (11 people). Many were linked to interactions with other epilepsy medicines and improved when doses were changed. Eight people (10.1%) stopped CBD by the last follow-up.
What does NICE say about CBD and epilepsy?
NICE recommends CBD with clobazam as an option for seizures linked to LGS (TA615) and Dravet syndrome (TA614), in people aged 2 and over. It also recommends CBD as an add-on option for seizures caused by tuberous sclerosis complex (TA873). Treatment should be reviewed every 6 months and stopped if seizures have not fallen by at least 30%. NICE’s guideline on cannabis-based medicinal products (NG144) refers to this guidance.
Is prescription CBD the same as CBD sold in shops?
No. The medicine in this study is a licensed, highly purified product made to pharmaceutical standards. CBD products sold in shops are not licensed medicines, and their content and quality can vary. CBD can interact with epilepsy medicines. Do not use shop CBD products alongside epilepsy treatment without talking to your neurology team.
Who carried out and funded the study?
Researchers at King’s College Hospital and King’s College London carried out the study. The lead author’s research fellowship was funded by the company that makes the licensed CBD medicine. The authors state the company had no role in designing the study, collecting or analysing the data, or writing the paper. The company reviewed the paper and could comment before publication. Several authors declared payments from this and other pharmaceutical companies. Curaleaf Clinic was not involved.
Thinking about treatment options?
Living with LGS, or caring for someone who has it, can be very demanding. If seizures are not well controlled, speak to your neurology team first. They can review current treatment and explain which options may suit. The charities Epilepsy Action and Epilepsy Society also offer advice and support for people with epilepsy and their carers.
The CBD medicine in this study is licensed for LGS and is available on the NHS through specialist epilepsy services. Most other cannabis-based medicines are unlicensed in the UK. A specialist doctor may consider them only after a full assessment, usually when licensed treatments have not helped enough. To find out whether a specialist consultation may be appropriate for you or someone you care for, you can complete Curaleaf Clinic’s online eligibility check. Any decision about treatment is made by a specialist doctor on an individual basis.