Attention-Deficit Hyperactivity Disorder – commonly referred to by the abbreviation “ADHD” – is a neurodevelopmental disorder that is typically characterised by three symptom categories: inattention, hyperactivity, and impulsivity. Other symptoms can include impairments in executive functioning, such as the ability to plan, organise, manage time, switch focus, or remember details, among others. The extent to which each of these symptoms presents varies from patient to patient. Furthermore, many patients with ADHD may also be at an increased risk of anxiety and mood disorders.
ADHD can have a significant impact on both social and professional aspects of a patient’s life. Evidence suggests that the presence of the disorder is often associated with lower socioeconomic status, higher rates of car accidents, problems with the law, and substance abuse. Diagnosis is usually identified in childhood years, though symptoms may be present in all life stages.
Treatment Options for ADHD
Current treatments for ADHD include stimulant and non-stimulant medications and cognitive behavioural therapy (CBT). Other treatments may be recommended for the treatment of certain symptoms and/or comorbidities. First-line treatments for ADHD are usually psychostimulants, but while these medications have been licensed for this indication due to their efficacy, they are often associated with unwanted side effects in some individuals, including insomnia, decreased appetite, and anxiety which may lead to poor treatment adherence.
However, attention has recently turned towards cannabis, and more specifically medical cannabis, for further research into its effects in individuals with ADHD. According to one study, in treatment-seeking patients who use cannabis, the prevalence of ADHD is around 34-46%. Another report states that the prevalence of adolescents with ADHD who use cannabis ranges from 33 to 38%.
Despite the apparent links between the presence of ADHD and cannabis use, few studies have examined the effects of cannabis on symptoms of ADHD. Existing literature has seemed equivocal with respect to the balancing of benefits and harms, with no clear answer on if medical cannabis is suitable for ADHD. Therefore, the authors of a recent study aimed to “systematically review the literature to synthesize our current understanding of the potential effects of cannabis on ADHD symptoms.”
Design and Methods of the Review
The authors identified relevant studies by conducting searches in MEDLINE, EMBASE, EMCARE, PsycINFO, Web of Science, Cochrane Central Registry of Controlled Trials, and Cochrane Database of Systematic Reviews. Grey Journal Pre-proof literature was searched in ClinicalTrials.gov. All relevant studies were included up to June 27th, 2022.
A broad inclusion criterion was applied to account for the scarcity of research on the topic. For example, studies and articles assessing the effects of cannabis on ADHD symptomology, including the effects of whole cannabis plant medicines, cannabinoids, synthetic cannabinoid-based products, and cannabis-derived cannabinoid-based products were included. Interventional studies, such as randomized controlled trials (RCTs) and open-label trials, as well as observational studies, such as cohort, cross-sectional, case-control, case series, and case report designs were included. Ineligible study designs included animal studies, literature reviews, and commentaries.
The studies were grouped based on whether they described the effect of ADHD on cannabis use or the effect of cannabis use on ADHD.
Results of the Review
A total of 39 eligible articles and studies were included in the final review. Of these, one randomised controlled trial employed an interventional design, while the others were observational studies. Among the observational studies, 23 were cross-sectional studies, seven were longitudinal studies, two were qualitative studies, and four were case reports. The majority (n = 24) assessed ADHD symptomology in relation to cannabis by comparing ADHD subjects with or without cannabis use. The remaining studies (n = 15) investigated neuropsychological functioning or the pathophysiology of ADHD by comparing cannabis users with or without ADHD.
The review found that ADHD patients with more severe symptoms had higher levels of cannabis use – a finding that supports previous reports. Among studies that compared ADHD subjects with or without cannabis use, results were mixed: some studies reported no association between cannabis use and ADHD symptoms, while others reported that cannabis use only worsens symptoms of inattention, hyperactivity, and impulsivity.
Furthermore, other studies found that reductions in cannabis use were associated with improvements in functioning and ADHD symptoms. However, the authors note that reductions in cannabis use are often associated with a reduction in the use of other substances, such as alcohol, which may mediate this apparent association.
Thirteen studies reported positive effects of cannabis use with ADHD symptoms. Results from case reports and case series indicated that cannabis (usually as an adjunctive therapy to standard treatments) may be associated with improved attention, impulsivity, concentration, and sleep in patients with ADHD. However, these studies were observational in nature and therefore it was not possible to prove that cannabis was responsible for these changes and randomised controlled trials are required to assess this further.
The conflicting evidence demonstrated in this systematic review suggests that many factors may modulate the effects of cannabis in ADHD. For example, the broad inclusion criteria applied to this review resulted in the inclusion of studies using a diverse range of cannabis preparations and administration methods. Moreover, most studies assessed in this review did not report the THC:CBD content of the cannabis products used by their participants.
The authors note that the findings indicated by this review are limited due to the lack of high-quality evidence in the existing literature. As such, it is recommended that further investigation be carried out to better understand the relationship between cannabis use and the presence of ADHD.