A systematic review found insufficient evidence to recommend either THC or CBD for treating schizophrenia | Research

The takeaway

A systematic review of 11 controlled trials found insufficient evidence that THC or CBD improves symptoms, cognition, or brain function in schizophrenia, with substantial variability across study designs.

Psychiatrists treating schizophrenia, patients with psychotic disorders considering cannabis, and researchers designing future cannabinoid trials in psychosis.

Insufficient evidence for THC or CBD in schizophrenia across 11 trials

What the researchers found

Intravenous THC increased psychosis in stable patients. Two reports on smoked/oral THC in patients with co-occurring CUD found no symptom changes but improved resting-state brain function. CBD 800 mg was as effective as amisulpride for acute psychosis. CBD augmentation results were mixed: 600 mg was not better than placebo, but 1,000 mg reduced symptoms in a sample that included cannabis users.

Why it matters

Despite growing public interest in cannabis-based treatments for mental illness, this review highlights that evidence remains insufficient and highly variable, with THC potentially worsening psychosis and CBD showing mixed results.

The numbers in context

11 reports reviewed. THC doses: intravenous (1 trial), smoked/oral (2 reports). CBD doses: 600 mg (not effective), 800 mg (equivalent to amisulpride), 1,000 mg (reduced symptoms). Substantial heterogeneity in dose, delivery, and patient selection.

How the study worked

Systematic review of 11 eligible reports identified from 8 online databases. Studies included placebo-controlled trials and neuroimaging studies using defined doses of THC or CBD in patients with schizophrenia or schizophrenia spectrum disorders.

What this study cannot tell us

Only 11 reports met inclusion criteria. Studies varied widely in dosing, duration, patient populations, and whether cannabis users were included or excluded. Most studies had small samples. Systematic review but not a quantitative meta-analysis due to heterogeneity.

How to read the evidence

Systematic review of controlled trials, but limited by small number of eligible studies and substantial heterogeneity in designs.

When this study was published

2021 systematic review. Reflects the state of evidence from trials through early 2021.

The bigger picture

The contrast between public enthusiasm for cannabis in mental health and the thin clinical evidence base underscores the need for well-designed, adequately powered trials before medical cannabis can be recommended for psychotic disorders.

Questions still open

Would larger, longer CBD trials with standardized doses show clearer effects? Should patients with CUD be studied separately from those without? Could CBD help as an adjunct specifically for patients who also use cannabis?

Common questions

Can CBD help schizophrenia?

Results were mixed. CBD 800 mg matched an antipsychotic in one acute trial, and CBD 1,000 mg reduced symptoms in another. But CBD 600 mg was not better than placebo. The reviewers concluded evidence is insufficient to recommend CBD for schizophrenia.

Is THC dangerous for people with schizophrenia?

In one trial, intravenous THC increased psychotic symptoms in stable patients. The review supports caution about THC use in people with psychotic disorders.

Read the original research

The Impact of THC and CBD in Schizophrenia: A Systematic Review.

Frontiers in psychiatry, 12, 694394

View on PubMed ↗Publisher’s paper ↗

Citation

Ahmed, Saeed; Roth, Robert M; Stanciu, Corneliu N; Brunette, Mary F. (2021). The Impact of THC and CBD in Schizophrenia: A Systematic Review.. Frontiers in psychiatry, 12, 694394. https://doi.org/10.3389/fpsyt.2021.694394