CBD

What it is

CBD (cannabidiol) is a non-intoxicating compound from the cannabis plant, sold over the counter in many countries as oils and capsules [1] [2]. Many over-the-counter cannabinoid products, including CBD oils, are unregulated [3].

How it might work

One proposed mechanism is that CBD slows the breakdown of anandamide, a natural compound that acts on the same receptors as THC [4]. CBD is also processed by the same liver enzymes (cytochrome P450) as many prescription medicines, which is the basis for interaction concerns [3].

What the research says

Anxiety. A 2024 meta-analysis of 8 trials (316 people) found CBD lowered anxiety scores with a large average effect (Hedges' g -0.92, 95% CI -1.80 to -0.04), though the authors caution the sample is small [5]. A 178-person phase 3 trial of a nano-dispersed CBD solution reported GAD-7 anxiety scores falling 7.02 points more than placebo over 15 weeks (95% CI -7.52 to -6.52); the product's developer ran the trial and employed some of the authors [6]. In a small trial in people recovering from heroin use disorder, 400-800 mg of CBD a day for 3 days reduced drug cue-induced anxiety [1]. However, a 2026 meta-analysis of 54 trials (2,477 people) found no significant effects on anxiety outcomes and rated most evidence low certainty [7], and a 2022 trial in 80 patients with social anxiety disorder or panic disorder found 300 mg of CBD did not improve exposure therapy outcomes [4].

Sleep. A 2025 meta-analysis of 6 trials (1,077 people) found cannabinoids improved self-reported sleep quality (SMD 0.53, 95% CI 0.03-1.02) but not when the product was CBD-only (SMD 0.13, 95% CI -0.38-0.65); the benefit came from other cannabinoids (SMD 0.82) [8]. In one trial of that kind, a 10:15 THC:CBD oil helped 60% of 29 adults with insomnia fall below the clinical insomnia threshold after two weeks, though the trial was small and blinding weakened in its second phase [9]. A 125-person crossover trial found 300 mg of CBD plus terpenes increased deep plus REM sleep by 1.3% of the night (95% CI 0.1-2.5), with no change in total sleep time; the maker funded it [10]. A 30-person pilot of 150 mg CBD found no difference in insomnia severity or most sleep measures, though objective sleep efficiency was better after two weeks [11]. A 2026 meta-analysis found cannabinoids increased sleep time in people with insomnia (SMD 0.54 by device, 0.55 by diary) [7].

Seizures. Prescription CBD is an add-on treatment for severe treatment-resistant childhood epilepsies. An umbrella review of meta-analyses rated it high certainty for at least halving seizure frequency (OR 0.59, 95% CI 0.38-0.92) [12], and a large meta-analysis of 152 randomised trials across conditions found CBD reduced seizure measures (SMD -0.5, 95% CI -0.62 to -0.38, high grade) [13]. Epilepsy trials averaged about 15 mg/kg a day, far above typical over-the-counter doses [14].

Pain. Pooled trials show modest pain relief from medicines that contain THC, such as nabiximols (SMD -0.25, 95% CI -0.37 to -0.14) [13], and an umbrella review rated cannabis-based medicines highly for chronic pain [12]. A review of CBD-only trials, mostly at low doses, found no clear effect on chronic pain [14].

Doses used in studies

Anxiety and sleep trials have used 150 mg nightly, 300 mg a day, and 400-800 mg a day for 3 days [11] [10] [4] [1]. Epilepsy trials averaged about 15 mg/kg a day [14]. A review of 58 randomised trials reported doses from 400 µg to 6,000 mg a day, reflecting how unstandardised dosing still is [15].

Safety and interactions

Most side effects are mild to moderate; the most common are diarrhoea, sleepiness, sedation and upper respiratory symptoms [16]. Pooled across 54 trials, cannabinoids increased the odds of any side effect (OR 1.75, 95% CI 1.25-2.46; about 1 extra person in 7) but not of serious side effects or dropouts [7]. CBD raised the odds of liver enzyme rises (OR 5.85, 95% CI 3.84-8.92) and drug-induced liver injury (OR 4.82, 95% CI 2.46-9.45); risk was concentrated at high doses (1,000 mg a day or more, or 20 mg/kg and above) and with antiepileptic drugs, with no cases seen in adults under 300 mg a day and no severe cases [17]. CBD is processed by the same liver enzymes as many medicines; case reports link cannabinoids to changed blood levels of warfarin, valproate, tacrolimus and sirolimus, with problems including bleeding and sedation [3]. Warfarin is the blood thinner with reported interactions, mostly at very high cannabinoid doses [18]. Serious side effects in trials have mostly involved people also taking clobazam or valproate [19]. Health organisations advise avoiding cannabis and CBD during pregnancy and breastfeeding, because data on CBD alone are very limited [20].

Caveats and open questions

The evidence is early: trials are small and short, and 24 of 54 trials (44%) in a 2026 review had a high risk of bias [7]. Expectations can shape results — in two studies, people given CBD-free oil but told it contained CBD showed changes in stress and anxiety responses [PMID 33813611] [PMID 38214314]. Only 2 of 34 studies in one insomnia review included people who actually had insomnia [2], and long-term safety data are limited [16]. Larger trials are under way, including a 519-person phase 3 trial of CBD capsules for insomnia and a phase 3 trial in generalised anxiety disorder.