Over-the-counter cannabidiol doses do little, but can still interact

A review in the British Journal of Clinical Pharmacology, published in July, examined pharmacokinetic, pharmacodynamic and clinical studies of low-dose cannabidiol (CBD), defined as up to 2.5 mg/kg or 175 mg a day, the range typically sold without prescription.

At these doses, peak and total exposure were 5 to 100 times lower than with a typical 20 mg/kg dose of the licensed product. Healthy volunteer studies showed little physical or neurological effect. Randomised trials found no benefit in Parkinson's disease, Crohn's disease, pain, fibromyalgia, stress, ocular hypertension or psoriasis, and only scattered, inconsistent improvements in sleep, alcohol use disorder and multiple sclerosis. Serious adverse events were not reported.

The pharmacologically important exception is interactions. Even at doses of 10 to 100 mg, interactions with THC, amitriptyline and hydromorphone were observed. CBD inhibits several CYP enzymes, so a patient on narrow-margin drugs who adds a CBD product may change their exposure without the prescriber knowing.

• Ask specifically about CBD oils and gummies when taking a drug history.
• Tell patients that low-dose CBD has little evidence of benefit for most conditions it is marketed for.
• Review co-prescribed drugs with narrow therapeutic ranges, such as tricyclics and opioids, in CBD users.
• Consider drug levels where available if a stable patient's response changes after starting CBD.

Why it matters

Patients think of CBD as inert; pharmacologically it can still shift the exposure of drugs they are prescribed.

Don't overread it

The interaction data come from small studies; the size of the effect in routine practice is unknown.

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