Cannabidiol

What it is

Cannabidiol (CBD) is one of the main substances in the cannabis plant. Unlike THC, the substance that makes people high and can bring on psychosis, it does not activate the brain’s main cannabinoid receptor. It slows the breakdown of anandamide, a cannabinoid the body makes itself, and in the first trial in schizophrenia, people whose anandamide rose more improved more.¹

How well it works

In that first trial, in Cologne, 42 people in hospital with acute schizophrenia took either cannabidiol or the antipsychotic amisulpride for four weeks. Both groups improved to a similar degree, and cannabidiol caused fewer movement side effects, less weight gain and no rise in the hormone prolactin.¹ Added to antipsychotics in 88 people for six weeks, it eased hallucinations and delusions slightly more than placebo.² In 36 outpatients with long-standing illness, a lower dose did not help and caused more sleepiness.³ Pooled across eight trials with 288 people, cannabidiol had no clear effect on psychotic symptoms.⁴

In people with schizophrenia who also used cannabis heavily, a dose of cannabidiol taken before cannabis did not protect against its effects; memory and psychotic symptoms were worse.⁵

Side effects

In the trials, side effects were about as common as on placebo or the comparison medicine; sleepiness was the most frequent.^4,3

Trials in Europe

STEP-ENHANCE, led from Oxford, is testing cannabidiol in 250 young people whose first episode of psychosis has not responded well to an antipsychotic.⁶ In July 2026, it was authorised in Austria, Germany, Greece, Italy, the Netherlands and Spain.⁷

What we don’t know yet

• Whether cannabidiol helps people with a first episode of psychosis, as STEP-ENHANCE is testing.
• Which dose, if any, is effective.