Endometriosis affects up to 1 in 10 women of reproductive age, yet treatment options are limited and the pain is often hard to control. Researchers have been asking whether cannabis-based medicines have any role to play, but good-quality evidence is still scarce.
A new analysis from the UK Medical Cannabis Registry followed 101 women prescribed medical cannabis for endometriosis over two years. This article explains what the study looked at, what it reported, and, just as importantly, what it cannot tell us.
Key Takeaways:
- The study: Researchers analysed data from 101 women prescribed cannabis-based medicinal products (CBMPs) for endometriosis, recorded in the UK Medical Cannabis Registry and followed for 24 months.
- Patient-reported outcomes. Over two years, patients reported improvements in pain, quality of life, sleep and anxiety compared with their starting scores. Because there was no comparison group, these changes cannot be attributed to the medicine alone.
- Prescribed opioids. Average prescribed opioid doses fell over the two years. Around a quarter of those ever prescribed opioids reported a meaningful reduction.
- Tolerability. Most patients (about 82%) reported no side effects. Among those who did, most events were mild or moderate, but a small number of severe events and one life-threatening event were recorded.
- What it means. These are encouraging real-world observations, but this was an observational study with no control group. Randomised controlled trials are still needed before medical cannabis could be recommended for endometriosis.
What is endometriosis?
Endometriosis is a long-term (chronic) condition in which tissue similar to the lining of the womb grows in other parts of the body, such as the ovaries, fallopian tubes, the outer surface of the womb and the lining of the pelvis. Less often, it can affect the bowel, bladder and other organs.
The condition can cause painful periods, ongoing pelvic pain, pain during sex, fatigue and, in some cases, difficulty getting pregnant. Its cause is not fully understood, and several theories have been proposed. Despite affecting so many women, a diagnosis can take a long time to reach. In the UK it is commonly cited as taking around eight to nine years on average.
How is endometriosis usually treated?
There is currently no cure for endometriosis, so treatment focuses on managing pain and other symptoms. Options usually include:
- Pain relief: non-steroidal anti-inflammatory drugs (NSAIDs), and in some cases opioids, although the evidence supporting their use in endometriosis is limited and they are not always suitable for long-term use.
- Hormonal therapies: treatments that lower oestrogen levels to slow the cyclical growth of womb-like tissue.
- Surgery: an operation to remove areas of endometriosis tissue.
These treatments do not work well enough for everyone, which is one reason researchers have looked at other options.
Why are researchers studying medical cannabis for endometriosis?
Cannabis-based medicines contain compounds called cannabinoids, the best known being cannabidiol (CBD) and tetrahydrocannabinol (THC). These interact with the body’s endocannabinoid system, which helps to regulate processes including how we sense pain.
Most of the direct evidence so far comes from laboratory and animal research. In animal studies, CBD has reduced inflammation and some types of pain, and THC has reduced pain sensitivity in models of surgically-induced endometriosis. These are early findings in animals. They do not show that cannabis-based medicines treat endometriosis in people, and there are still no completed randomised controlled trials in this area.
What is the UK Medical Cannabis Registry?
Cannabis-based medicines were legalised in the UK in November 2018, allowing specialist doctors to start them in specific circumstances. Because these medicines remain unlicensed for most conditions, real-world data on how patients get on with them is valuable.
Curaleaf Clinic set up the UK Medical Cannabis Registry in December 2019 to collect this kind of information from patients prescribed medical cannabis for a range of conditions. The aim is to build the evidence base on the safety and effects of medical cannabis, so that future clinical practice and research can be better informed. The full study is published in the Australian and New Zealand Journal of Obstetrics and Gynaecology.
What did this study look at?
Researchers analysed data from women (aged 18 and over) with a main diagnosis of endometriosis who had been enrolled in the registry for at least two years. Patients completed a set of validated questionnaires, known as patient-reported outcome measures (PROMs), at the start of treatment and again at 1, 3, 6, 12, 18 and 24 months. Any side effects (adverse events) were recorded by patients or clinicians.
The questionnaires covered pain, quality of life, anxiety and sleep:
- Brief Pain Inventory (BPI) short form: measures how severe pain is and how much it interferes with daily life.
- Short Form McGill Pain Questionnaire-2 (SF-MPQ-2): measures the intensity and quality of pain.
- Pain Visual Analogue Scale (Pain VAS): a simple scale from “no pain” to “worst pain”.
- Patient Global Impression of Change (PGIC): a single question on how much a person feels their condition has changed.
- EQ-5D-5L: a general quality-of-life measure covering mobility, self-care, usual activities, pain and mood.
- Generalised Anxiety Disorder-7 (GAD-7): a seven-item measure of anxiety symptoms.
- Single-Item Sleep Quality Scale (SQS): rates sleep quality over the past week.
Who took part?
A total of 101 women were included, with an average age of 35.5 years. Most had used cannabis before starting treatment, either currently (42 women, around 42%) or in the past (30 women, around 30%). This matters when reading the results, because people who choose to try medical cannabis may differ from the wider population of women with endometriosis.
What did the study find?
Pain, quality of life, sleep and anxiety
Over the 24 months, patients reported improvements from their starting scores across measures of pain (BPI severity and interference, the SF-MPQ-2 and the Pain VAS), overall quality of life (EQ-5D-5L), sleep (SQS) and anxiety (GAD-7). The authors describe these improvements as sustained across the two years.
The table below summarises the direction of change reported for each measure.
| What was measured | Questionnaire | Reported change over 24 months |
| Pain severity | BPI Severity | Improved from baseline |
| Pain interference with daily life | BPI Interference | Improved from baseline |
| Pain intensity and quality | SF-MPQ-2 Total | Improved from baseline |
| Overall pain rating | Pain VAS | Improved from baseline |
| General quality of life | EQ-5D-5L Index | Improved from baseline |
| Anxiety symptoms | GAD-7 | Improved from baseline |
| Sleep quality | SQS | Improved from baseline |
These are group averages from questionnaires. They show an association between starting treatment and a change in scores. They do not, on their own, prove that the medicine caused the change, because there was no untreated comparison group.
Prescribed opioids
At the start, patients were prescribed an average of 19.9 mg a day of opioids (measured as oral morphine equivalent). By 24 months, this had fallen to an average of 14.8 mg a day. Among the 46 patients prescribed opioids at any point, around a quarter (26.1%) reported a clinically meaningful reduction in their opioid dose by 24 months.
Side effects
Most patients tolerated treatment well. Eighty-three of the 101 patients (about 82%) reported no side effects at all. Eighteen patients (around 18%) reported a total of 165 events between them.
Of those 165 events, about half were mild (84 events, 50.9%) and most of the rest were moderate (69 events, 41.8%). Eleven events were classed as severe, and one was recorded as life-threatening. The most commonly reported side effects were fatigue, lethargy, headache, dry mouth and difficulty sleeping. This balance matters: “well tolerated in a study” is not the same as “safe for everyone”, and any medicine can carry risks.
What are the limitations of this study?
The authors are clear that this study has important limits. The main points to keep in mind are:
- No control group. There was no comparison group of untreated patients, so the changes cannot be separated from natural ups and downs in the condition, regression to the mean, or the placebo effect.
- Observational design. The study can show associations, but it cannot prove that medical cannabis caused the changes.
Who took part. Patients came through a private clinic and were mostly younger women, many of whom had used cannabis before. Results may not apply to everyone with endometriosis. - Different products and doses. Patients used a range of formulations, doses and THC-to-CBD ratios, so the results cannot be pinned to any single product or dose.
- Drop-out over time. Not everyone completed every questionnaire, and people who felt better or worse may have been more or less likely to keep taking part.
What does this tell us?
This is one of the larger and longer real-world analyses of medical cannabis in endometriosis to date, and it adds to an earlier registry analysis that followed patients for six months. It reports improvements in pain, quality of life, sleep and anxiety that were sustained over two years, alongside a mostly mild-to-moderate side-effect profile.
It does not show that medical cannabis treats endometriosis, or that it is right for any individual. Without a control group, the improvements cannot be attributed to the medicine with confidence. As the authors conclude, randomised controlled trials are essential to establish whether cannabis-based medicines are effective, what doses might be best, and how safe they are over the long term, before they could be recommended as a treatment option for endometriosis.
Frequently Asked Questions
What is endometriosis?
Endometriosis is a chronic condition in which tissue similar to the lining of the womb grows outside the womb, such as on the ovaries, fallopian tubes and pelvic lining. It can cause painful periods, ongoing pelvic pain, pain during sex, fatigue and, in some cases, fertility problems.
What are the treatments for endometriosis?
There is no cure. Treatment focuses on managing symptoms and usually includes pain relief (such as NSAIDs and sometimes opioids), hormonal therapies that regulate hormone levels across the menstrual cyucle, and surgery to remove endometriosis tissue. Treatment is tailored to the individual.
Can medical cannabis be prescribed for endometriosis in the UK?
Cannabis-based medicines have been legal to prescribe in the UK since November 2018, but can only be stated by a specialist doctor and only in specific circumstances, usually when licensed treatments have not helped enough. They remain unlicensed for endometriosis, and any decision is made by a specialist on a case-by-case basis.
What did this study find about side effects?
Most patients (about 82%) reported no side effects. Among those who did, most events were mild or moderate, though a small number of severe events and one life-threatening event were recorded. The most common side effects were fatigue, lethargy and headache.
Does this study prove medical cannabis works for endometriosis?
No. It is an observational study without a control group, so it can show associations but cannot prove cause and effect. The authors call for randomised controlled trials to test whether these medicines are effective and safe.
Thinking about your treatment options?
In the UK, cannabis-based medicines are prescription-only and remain unlicensed for endometriosis. They are considered only by a specialist doctor, after a full assessment, and only when licensed treatments have not provided enough relief.
If you would like to understand whether a specialist consultation may be appropriate for you, you can complete Curaleaf Clinic’s online eligibility check. Any decision about treatment is made by a specialist doctor on an individual basis.
