THC impaired the brain's ability to stop unwanted responses, and the impairment correlated with psychotic symptoms | Research

The takeaway

In 36 healthy men, THC caused more errors on a stop-signal task and reduced activation in a brain region critical for impulse control, with both effects correlating with the severity of transient psychotic symptoms.

Read this if you want to understand the brain mechanisms linking THC to psychotic-like experiences.

THC-induced inhibition deficits correlated with psychotic symptom severity

What the researchers found

Researchers gave 36 healthy men with minimal cannabis exposure either 10mg of oral THC or placebo in a double-blind crossover design, then scanned their brains during a task requiring them to withhold automatic responses (Go/No-Go task).

THC increased inhibition errors and reduced inhibition efficiency compared to placebo. It also reduced activation in the left inferior frontal gyrus, a brain region critical for stopping unwanted actions. The severity of transient psychotic symptoms induced by THC directly correlated with more frequent inhibition errors and lower inhibition efficiency.

The left inferior frontal activation was inversely correlated with both error frequency and psychotic symptom severity, suggesting that THC's disruption of this brain region may be a mechanism linking cannabis to transient psychotic experiences.

Why it matters

This study provides experimental evidence that THC disrupts the brain's ability to suppress unwanted responses, and that this specific cognitive impairment is directly related to the psychotic symptoms that THC can temporarily induce. This mechanistic link helps explain why cannabis use is associated with psychosis risk.

The numbers in context

36 healthy men studied in crossover design. 10mg oral THC vs. placebo. THC increased inhibition errors and reduced inhibition efficiency. Left inferior frontal activation was reduced by THC. Psychotic symptom severity correlated with error frequency.

How the study worked

Randomized, placebo-controlled, double-blind, within-subject crossover design. 36 healthy right-handed men with minimal cannabis exposure received 10mg oral delta-9-THC or placebo on separate sessions. Brain activation was measured with fMRI during a Go/No-Go response inhibition task.

What this study cannot tell us

The study used healthy men with minimal cannabis exposure, so results may not generalize to regular users who may develop tolerance. Oral THC administration differs from smoked cannabis. 10mg is a specific dose and effects may vary at other doses. The crossover design means order effects are possible.

How to read the evidence

Randomized, placebo-controlled crossover design with brain imaging in 36 participants. Strong experimental design for a cannabis neuroscience study.

When this study was published

Published in 2015 using a well-controlled experimental design.

The bigger picture

The ability to inhibit thoughts and actions is fundamental to organized behavior. This study shows THC specifically disrupts this capacity in a brain region also implicated in schizophrenia, providing a concrete neural mechanism for how cannabis might contribute to psychotic experiences.

Questions still open

Would regular cannabis users show the same inhibitory deficits and brain changes? Does CBD counteract THC's effects on inhibitory control? Is the left inferior frontal disruption reversible with cessation?

Common questions

Does THC cause psychosis?

This study showed that THC can temporarily induce psychotic symptoms in healthy people and that these symptoms are linked to specific impairments in inhibitory brain function. This does not mean everyone who uses THC will develop psychosis, but it reveals a neural mechanism for the association.

What is inhibitory control?

Inhibitory control is the brain's ability to suppress unwanted thoughts, impulses, and automatic responses. It is essential for goal-directed behavior and is disrupted in several psychiatric conditions, including schizophrenia.

Read the original research

Impairment of inhibitory control processing related to acute psychotomimetic effects of cannabis.

European neuropsychopharmacology : the journal of the European College of Neuropsychopharmacology, 25(1), 26-37

View on PubMed ↗Publisher’s paper ↗

Citation

Bhattacharyya, Sagnik; Atakan, Z; Martin-Santos, R; Crippa, J A; Kambeitz, J; Malhi, S; Giampietro, V; Williams, S; Brammer, M; Rubia, K; Collier, D A; McGuire, P K. (2015). Impairment of inhibitory control processing related to acute psychotomimetic effects of cannabis.. European neuropsychopharmacology : the journal of the European College of Neuropsychopharmacology, 25(1), 26-37. https://doi.org/10.1016/j.euroneuro.2014.11.018