Small Australian Trial Finds Little Short-Term Cognitive Change After Prescribed Medical Cannabis
A small Australian study found little evidence of short-term cognitive decline after experienced medical-cannabis patients took their prescribed products—but the results do not establish that cannabis is free of impairment or risk.
Published in 2023 in CNS Drugs, the open-label trial included 40 adults who were already using prescribed medical cannabis for conditions including chronic non-cancer pain, sleep disorders and anxiety. Participants self-administered their usual pharmacy-labeled dose in a laboratory session. Twenty-three used oral oils and 17 vaporized cannabis flower.
Researchers assessed participants before treatment and again afterward using computerized tests of multitasking, visual information processing, pattern-recognition memory, reaction time and spatial working memory. Testing took place about three hours after administration for the main cognitive battery, with an additional driving-related performance assessment at roughly three and 5.5 hours.
Most measures did not change significantly after treatment. Scores on the multitasking and rapid-visual-information-processing tests improved over time, while the other measures showed no statistically significant differences. Because the study did not include a placebo group, those improvements may partly reflect practice effects rather than a beneficial effect of cannabis. The researchers concluded that prescribed medical cannabis appeared to have a minimal acute effect on cognition in this sample, while emphasizing that larger controlled studies are needed.
The type of product did appear to affect how participants felt. Those who vaporized flower reported stronger feelings of being “stoned” and sedated than participants who used oils. That distinction matters: a person may perform normally on a limited set of computerized tests while still feeling intoxicated, drowsy or otherwise unfit for activities requiring full attention.
The findings also cannot be generalized to everyone who uses cannabis. Participants had already been taking medical cannabis for an average of about 10 months, so tolerance and familiarity with their products may have influenced the results. The trial examined a single dose under supervised conditions, rather than long-term use in everyday life. It also did not screen participants for every possible substance that could affect performance, and the sample was small.
A systematic review of medical-cannabis research reached a similarly cautious conclusion: 15 of 23 included studies found no significant cognitive effect, but six reported adverse effects. The studies varied substantially in patient groups, products, doses, treatment duration and cognitive tests, making broad conclusions difficult.
For patients, the practical message is not that prescribed cannabis cannot impair them. Effects depend on the product, THC content, dose, route of administration, tolerance, other medicines and the underlying health condition. The Therapeutic Goods Administration’s patient guidance advises that driving while impaired by medicinal cannabis is unlawful and that THC may remain detectable after impairment has subsided. Healthdirect likewise advises people not to drive or operate machinery while using THC-containing medicinal cannabis.
Patients starting treatment or increasing a dose should follow their prescriber’s instructions, allow time to understand how the product affects them and avoid alcohol or other sedating substances unless their clinician says otherwise. Anyone who feels drowsy, slowed, dizzy or cognitively affected should not drive or perform safety-sensitive work.
Overall, the 2023 trial offers limited reassurance for established medical-cannabis users: under the conditions tested, one prescribed dose was not associated with measurable deterioration across most cognitive tasks. It does not rule out impairment in new users, after dose increases, with higher-THC products or during longer-term treatment. More rigorous placebo-controlled studies with larger and more diverse patient groups are still needed.
