Cannabis is classified as a class B drug in the UK with penalties for possession of up to 5 years in prison, an unlimited fine or both. Nevertheless it is widely available and is the most commonly used drug in the UK with approximately 2.6 million (7.6%) of adults reporting that they sometimes or regularly use it. It is not uncommon for people who present in our epilepsy clinic to report regular use of cannabis; some use it recreationally whilst others report ‘self-medicating’ based on the belief that it has a beneficial impact on their seizures. The aim of this study was to establish the prevalence of cannabis use in people with epilepsy referred for a neuropsychological assessment and to examine the impact of cannabis use on cognitive function in this group.

All patients who attend for a neuropsychological assessment are routinely asked about illegal drug use in their clinical interview. This information is also captured in the medical and neuropsychiatric assessments they undergo when assessed by the multidisciplinary team. The electronic medical records of 800 consecutive patients who had undergone a neuropsychological assessment between 2019 and 2022 were searched for references to cannabis use. The neuropsychological profiles of patients reporting cannabis use were compared to those seen in the larger series across multiple cognitive domains.

Seventy (8.75%) of the patients in the series reported past or present cannabis use. Cannabis users were more likely to be male (p<0.01) and were younger (p<0.01) than those who did not report use. Reading IQ was significantly lower in the cannabis group (p<0.001). Patients who were regularly using cannabis at the time of the neuropsychological assessment did not differ from the rest of the cohort on tests of processing speed, working memory, naming or verbal fluency. There were no differences between the groups in their performance on an embedded measure of performance validity. However the patients who were regularly using cannabis at the time of their neuropsychological assessments scored significantly lower on tests of verbal learning (p<0.05) and reported significantly greater subjective memory problems in everyday life (p=0.02) than the non-cannabis group. The group using cannabis also scored significantly more highly on the depression (p<0.01) and anxiety scales (p=0.02) on the Hospital Anxiety and Depression Scale.

The prevalence and patterns of cannabis use in the epilepsy population mirror those seen in the wider population. The impact on regular cannabis use on neuropsychological function appears to be most evident on measures of new learning and subjective measures of memory disturbance. Cannabis use is significantly associated with lower levels of cognitive reserve and elevated levels of anxiety and low mood. Whilst caution must be employed with respect to any direct attribution in these complex clinical presentations, these findings may be helpful in the interpretation of neuropsychological test scores and the planning of interventions, particularly with respect to subjective memory complaints in this group.