Cannabis and ADHD
Cannabis is the most commonly used illegal drug in the developed world. Many, if not most, of my young adult patients with ADHD smoke cannabis and believe that it helps them. Nevertheless, they are usually coming to see me because they are not doing well in some way. Usually they are either unemployed or underemployed. Often they seem to be drifting aimlessly without a clear goal or direction in life. Many are overly involved in video games and spend most of their waking hours in front of their computers even while they should be out looking for work. How does one explain this discrepancy between their perception and the objective state of their lives? I suspect they are confusing the fact that cannabis may make them care less, or feel mellow, with the effects it is having on their cognitive, coping and problem solving abilities.
ADHD is characterized by problems with such things as executive functioning, response inhibition, concentration, attention, memory and impulse control. Before accepting that cannabis is helping, I want to see evidence that it improves these abilities. I have seen no such evidence. Usually, if I can convince them to stop the cannabis they admit that they are functioning and even feeling better once they have made the transition to a cannabis-free state of mind.
Research on the neuropsychological effects of cannabis in community samples of moderate users has consistently shown negative effects on verbal learning, memory and attentional functions. Among heavy users the effects are even greater and have been shown to persist for weeks after discontinuation of cannabis. There is also reason to believe, from both animal and human studies that the effects are greater on adolescents, whose brains are still developing, than on adults.
Brain imaging studies have also shown that heavy use of cannabis is associated with structural brain abnormalities (smaller volume of hippocampus and amygdala) in both humans and animals. Early onset cannabis users (before age 17) were found to have smaller whole brain volumes, lower percent cortical grey matter, and higher percent white matter.
A comprehensive review of the literature on the effects of cannabis on memory, concluded that impaired memory persists beyond the period of acute intoxication. Deficits increase with the frequency, duration, dose and age of onset of cannabis use. Deficits persist for weeks after the discontinuation of cannabis but whether they are permanent has not yet been determined.
It would be very interesting to do a controlled, double-blind, placebo controlled trial of cannabis for ADHD, to settle this question definitively. Such a study would be very feasible and easy to recruit for. I would have no trouble at all finding willing participants for such a trial. Until such time as I see such results, I have no basis for recommending cannabis for ADHD. It seems, from the available evidence to be contraindicated.