In this study of Dutch primary brain tumor patients, a little over half of patients reported past or current cannabis use, with 14% actively using it. Of all brain tumor patients currently using cannabis, 57% used it for symptom relief and 43% used it for presumed effects on their tumor.

The percentage of current cannabis users in our cohort was comparable to that of Dutch cancer patients undergoing systemic anticancer treatment,¹² but higher than in the general population. In 2021, an estimated 1.3% of European adults aged 25–64 used cannabis daily, and in the Netherlands, 6.4% of adults used it in the last month.⁷ Cannabis use in the Netherlands seems less prevalent than in the USA, where 14% of adults aged 26 or older used cannabis in the past month.¹³ Similar to these differences in the general population, the percentage of current cannabis users was lower in our cohort than among 73 glioma patients in Florida, where 22% currently used cannabis.² Although cannabis use among Dutch patients seems less prevalent than in the USA, cannabis users still comprise a significant portion of patients.

In a comparable German study of 102 patients with an intracranial glioma, 14% used alternative therapies, although it is unclear if this included cannabis or other substances.¹⁴ Another survey among 277 French glioma patients found that 9% used medication without a prescription, and 22% used a food supplement.¹⁵ The authors did not specify the nature of these substances, but cannabis could potentially fall under either category. To our knowledge, no specific data exist on the use general complementary therapies among Dutch neuro-oncology patients.

Interestingly, a significant percentage of patients, including tumor-related users, first used cannabis before their tumor diagnosis. This could be attributed to the more relaxed regulations in the Netherlands and the high rate of lifetime cannabis experimentation, which stands at 28.8% in the general population.⁶ It is plausible that patients who had previously tried cannabis were more open to using it again following their brain tumor diagnosis.

Of all brain tumor patients currently using cannabis, most people use it for symptom relief. The most commonly reported improvements were in sleep, anxiety, worrying, and depressive symptoms. Patients reported cannabis use to be beneficial with one patient reporting only a little worsening of cognition and appetite, whereas most other symptoms either improved a little or improved a lot. Of note, some answers were missing. Possibly, patients did not experience a specific symptom such as epilepsy or cognitive problems prior to starting cannabis so therefore did not know cannabis’ effect on it, did not remember the effect if they were prior users, or had a lack of time as they completed the questionnaire in the waiting room. Nevertheless, our findings align with the findings in the Florida glioma cohort, where patients also reported an improved ability to cope emotionally, to relax, and to sleep. However, that study only reported 7 symptoms and they did not report if any symptoms worsened.²

Our findings on perceived improvements in sleep corroborate a recent meta-analysis in patients with sleep disturbances, which found modest evidence for small benefits of cannabis on sleep quality and sleep disturbance among chronic pain patients.¹⁶ However, they did not differentiate between different cannabis components.

Additionally, patients in our cohort frequently reported improvements in mental health symptoms, ie, anxiety, worrying, and depressive symptoms. This corroborates a 2019 meta-analysis among patients with various diseases¹⁷: THC did not affect depressive symptoms but improved anxiety, though the evidence was graded low-quality and the effects were small. Conversely, another meta-analysis shows that acute administration of THC induced both anxiety and depression in healthy volunteers.¹⁸ CBD, however, shows less conflicting results and might be promising in the management of anxiety based on animal research and some small studies in healthy volunteers and patients with an anxiety disorder.¹⁹

A significant part of current cannabis users also reported using it for presumed effects on their tumor. This is more than in the Florida glioma cohort, where only 3 out of 73 patients independently reported this reason,² but aligns with a Dutch study involving 153 patients with extra-cranial cancer, where 46% used cannabis for presumed antitumor effects.¹²

While some preclinical studies have suggested potential benefits on cell growth, apoptosis, and angiogenesis, clinical evidence remains insufficient with just one placebo-controlled clinical trial.^20,21 In the randomized, double-blind, and placebo-controlled part of this phase 1b study involving 21 patients with a recurrent glioblastoma, a CBD:THC spray alongside Temozolomide resulted in a more favorable survival.²¹ However, the small sample size, favorable demographics, 2 early deaths in the placebo group, and unavailable molecular information complicate the interpretation of the results.^4,21

In our cohort, few adverse events were reported, with the most commonly reported being drowsiness, dry mouth, and dizziness. Given the observational design of our study, the self-reported adverse events could possibly be attributed to other medications or treatments. Moreover, 21% of patients were undergoing chemotherapy at the time of questionnaire completion, and 7% were receiving chemoradiation, which may have also contributed to these reported adverse events. However, these findings are consistent with known adverse events of medical cannabis, which are common and include somnolence, dizziness, dry mouth, and nausea.²² The occurrence of adverse events might depend on various aspects such as the cannabis components and dosage of cannabis used. The few self-reported adverse events in our study could be due to the preferred use of CBD among participants. CBD has a more favorable adverse event profile than THC with few serious adverse events, and is generally well-tolerated.²³

We found that a considerable proportion of patients were unaware of the constituents of their cannabis. Of the patients using cannabis for tumor-related effects who did know the constituents, the majority favored CBD-dominant products. Doses of cannabis varied greatly among those who knew the dose of their cannabis product. These findings underscore the need for more comprehensive information regarding the distinct effects of both CBD and THC and of different doses.⁵