CBD did not block THC-induced psychotic symptoms or memory impairment when both were vaporized at a 2:1 ratio | Research

The takeaway

A randomized, double-blind crossover trial of 48 cannabis users found that vaporized THC (8 mg) increased psychotic symptoms and impaired memory, but co-administration of CBD (16 mg) at a 2:1 ratio did not attenuate these effects. CBD alone reduced psychotic symptoms only in light cannabis users.

Cannabis researchers studying CBD-THC interactions; psychiatrists treating cannabis-using patients with psychosis risk; consumers assuming CBD products protect against THC effects.

CBD at 2:1 ratio did not protect against any of THC's psychotic or cognitive effects

What the researchers found

Forty-eight cannabis users were selected based on schizotypal personality scores and use frequency, then received four vaporized conditions in crossover design: placebo, THC 8 mg, CBD 16 mg, and THC 8 mg + CBD 16 mg.

THC alone increased psychotic symptoms on the PSI, increased negative symptoms on the BPRS, and robustly impaired both episodic and working memory.

Co-administration of CBD at a 2:1 ratio (16 mg CBD with 8 mg THC) did not attenuate any of these THC effects.

CBD alone reduced PSI scores, but only in light cannabis users. Frequent users showed no benefit from CBD alone.

The authors noted that frequent cannabis users may show a blunted antipsychotic response to CBD, which is concerning given high rates of cannabis use disorders in schizophrenia patients.

Why it matters

The popular belief that CBD protects against THC's negative effects is not supported at this dose ratio when both are vaporized. This has direct implications for consumers who choose high-CBD products assuming they are "safer" and for the medical cannabis industry marketing CBD-containing products.

The numbers in context

48 cannabis users. THC 8 mg, CBD 16 mg (2:1 ratio). THC increased PSI scores, BPRS negative symptoms, and impaired episodic and working memory. CBD did not attenuate any THC effect. CBD alone reduced PSI only in light users.

How the study worked

Randomized, double-blind, crossover design. 48 cannabis users selected by schizotypal scores (low/high) and use frequency (light/heavy). Four vaporized conditions. Measures: BPRS, PSI, immediate and delayed prose recall, 1-back and 2-back tasks.

What this study cannot tell us

Single-dose acute study may not reflect chronic use patterns. Only one CBD:THC ratio (2:1) tested. Vaporized route may produce different dynamics than oral administration. Selection of participants by schizotypal scores and use frequency creates a specific sample.

How to read the evidence

Strong. Randomized, double-blind, crossover design with multiple validated outcome measures and stratified participant selection.

When this study was published

Published in 2018. The question of whether CBD protects against THC's adverse effects continues to be actively investigated with varying results.

The bigger picture

This challenges the simple narrative that "CBD counteracts THC." The reality appears more nuanced: the ratio, route, dose, and the user's prior cannabis history all matter. For frequent users, CBD may lose its protective properties entirely.

Questions still open

Would higher CBD:THC ratios be protective? Does chronic CBD co-administration work differently than acute? Why do frequent users lose CBD's antipsychotic benefit? Does tolerance to CBD develop?

Common questions

Does this mean CBD does not protect against THC at all?

At the 2:1 ratio tested via vaporization, no protection was observed. Other studies using different ratios, routes, or doses have found some protective effects. The relationship between CBD and THC is complex and dose-dependent, not a simple on/off switch.

Why did CBD only help light users?

Frequent cannabis users may develop tolerance to CBD's effects on cannabinoid signaling, or their endocannabinoid system may be altered in ways that reduce CBD's effectiveness. This is particularly concerning for heavy users who might most need CBD's protective properties.

Read the original research

Individual and combined effects of acute delta-9-tetrahydrocannabinol and cannabidiol on psychotomimetic symptoms and memory function.

Translational psychiatry, 8(1), 181

View on PubMed ↗Publisher’s paper ↗

Citation

Morgan, Celia J A; Freeman, Tom P; Hindocha, Chandni; Schafer, Grainne; Gardner, Chelsea; Curran, H Valerie. (2018). Individual and combined effects of acute delta-9-tetrahydrocannabinol and cannabidiol on psychotomimetic symptoms and memory function.. Translational psychiatry, 8(1), 181. https://doi.org/10.1038/s41398-018-0191-x