Nine published clinical studies, with a mix of large trials, small pilots and one with a null result. The summaries are simplified. Read the original paper for the full methods, numbers and limits.
2017Randomized, placebo-controlled
Trial of Cannabidiol for Drug-Resistant Seizures in the Dravet Syndrome
Devinsky O, et al. N Engl J Med
- What was tested
- Purified cannabidiol (CBD) oral solution versus placebo, added to existing medicines, over 14 weeks in 120 children and teenagers with Dravet syndrome.
- What researchers found
- Median monthly convulsive seizures went from 12.4 to 5.9 in the CBD group and from 14.9 to 14.1 in the placebo group. Side effects such as diarrhea, vomiting, fatigue, fever, sleepiness and abnormal liver tests were more common with CBD.
2018Randomized, placebo-controlled
Cannabidiol in patients with seizures associated with Lennox-Gastaut syndrome (GWPCARE4)
Thiele EA, et al. Lancet
- What was tested
- CBD (20 mg/kg/day) versus placebo, added to existing medicines, over 14 weeks in 171 patients with Lennox-Gastaut syndrome.
- What researchers found
- Monthly drop seizures fell by a median of 43.9% with CBD versus 21.8% with placebo. More people stopped early because of side effects in the CBD group (14 vs 1).
2021Randomized, placebo-controlled
Add-on Cannabidiol for Seizures Associated With Tuberous Sclerosis Complex (GWPCARE6)
Thiele EA, et al. JAMA Neurol
- What was tested
- Two CBD doses (25 and 50 mg/kg/day) versus placebo over 16 weeks in 224 patients with tuberous sclerosis complex.
- What researchers found
- Seizures fell by 48.6% (25 mg/kg) and 47.5% (50 mg/kg) versus 26.5% with placebo. The lower dose had a better safety profile. Liver enzyme elevations occurred in 18.9% of CBD-treated patients.
2007Small randomized trial
Cannabis in painful HIV-associated sensory neuropathy
Abrams DI, et al. Neurology
- What was tested
- Smoked cannabis (3.56% THC) versus placebo cigarettes for 5 days in an inpatient setting. 50 participants completed the study.
- What researchers found
- Daily pain ratings fell 34% with cannabis versus 17% with placebo. 52% vs 24% had at least a 30% reduction. The study was small and short, and it took place in a controlled clinical research unit.
2011Enriched-design randomized trial
Nabiximols (Sativex) as add-on for MS spasticity
Novotna A, et al. Eur J Neurol
- What was tested
- A THC/CBD oromucosal spray. 572 people with multiple sclerosis spasticity tried it first, and the 241 who responded early were then randomized to continue it or switch to placebo.
- What researchers found
- Spasticity scores favored nabiximols over placebo (P = 0.0002). Because only early responders were randomized, the result applies to that group, not to everyone who tries it.
2006Null result
Cannabis extract vs THC vs placebo in cancer-related anorexia-cachexia
Strasser F, et al. J Clin Oncol
- What was tested
- A cannabis extract, THC alone or placebo for 6 weeks in 243 randomized patients with advanced cancer and weight or appetite loss.
- What researchers found
- No differences between groups in appetite or quality of life. Recruitment was stopped early. This is a useful reminder that not every trial finds an effect.
2018Randomized, placebo-controlled
Cannabidiol as an adjunctive therapy in schizophrenia
McGuire P, et al. Am J Psychiatry
- What was tested
- CBD (1,000 mg/day) or placebo added to existing antipsychotic medicine for 6 weeks in 88 patients.
- What researchers found
- Fewer positive psychotic symptoms with CBD, and clinicians rated more patients as improved. Changes in cognition and overall functioning were not statistically significant. Side effects were similar between groups.
2019Exploratory randomized trial
Cannabidiol for cue-induced craving and anxiety in drug-abstinent heroin use disorder
Hurd YL, et al. Am J Psychiatry
- What was tested
- Single daily doses of CBD (400 or 800 mg) or placebo for 3 days in abstinent adults, followed by lab exposure to drug-related cues.
- What researchers found
- CBD reduced cue-induced craving and anxiety, plus heart rate and cortisol responses, with some effects lasting about a week. There were no serious side effects. It was a small exploratory trial, and the authors called for further investigation.
2020Phase 2a, adaptive design
Cannabidiol for the treatment of cannabis use disorder
Freeman TP, et al. Lancet Psychiatry
- What was tested
- CBD at 200, 400 or 800 mg/day versus placebo for 4 weeks during a quit attempt in 82 adults, all of whom also received brief motivational interviewing,, using an adaptive Bayesian design.
- What researchers found
- The 200 mg dose was dropped early as ineffective. The 400 mg and 800 mg doses reduced cannabis use compared with placebo. As a phase 2a trial, it was designed to guide larger studies, not to settle the question.