Rationale <p>People with attention-deficit/hyperactivity disorder (ADHD) are more likely to use cannabis. However, the effects of chronic and acute cannabis use on people with ADHD remain unclear.</p> Methods <p>104 adults (26 cannabis users with ADHD, 26 non-users with ADHD, 26 cannabis users without ADHD, and 26 non-users without ADHD) were recruited to assess the impacts of cannabis use on perceived ADHD symptoms and cognition, through a repeated-measures, quasi-experimental study. In session one, participants completed a battery of cognitive tests and provided ADHD symptom (impulsivity, hyperactivity, inattention) and affect (irritability, anxiety, mood) ratings. In session two, cannabis users were observed using their own cannabis product while non-users remained sober, prior to completing the same battery of cognitive tests and symptom ratings.</p> Results <p>Chronic cannabis users with ADHD self-reported less severe sluggish cognitive tempo than non-users with ADHD. Nevertheless, chronic cannabis users with (and without) ADHD demonstrated worse performance on two objective tests of cognition. Intoxicated cannabis users with ADHD self-reported reductions in impulsivity, irritability, and anxiety as well as enhanced mood, and demonstrated reductions in impulsive responding on two cognitive tests. Nevertheless, intoxicated cannabis users with (and without) ADHD demonstrated reductions in performance on five of the other cognitive outcomes.</p> Conclusion <p>The present study provides limited evidence that acute cannabis intoxication may selectively reduce subjective and objective impulsivity in people with ADHD, while impairing several other aspects of cognition. In contrast, chronic cannabis use appears to reduce perceptions of sluggish cognitive tempo without producing any objective benefits to cognition.</p>