CBD and nausea: what the research has examined

The licensed cannabinoid is not CBD

Two cannabinoid medicines have been licensed for nausea and vomiting caused by chemotherapy: dronabinol, which is synthetic THC, and nabilone, a synthetic analogue of it. Both work through the CB1 receptor — the one responsible for intoxication — and both are prescribed, with the side effects that go with that route.

This is the single most important fact about the topic and the one most often blurred. When an article says cannabinoids have been shown to reduce nausea, and links to a review of the licensed medicines, it is describing THC. A bottle of CBD is not a mild version of dronabinol; it is a different compound acting on a different receptor.

What the CBD work has actually shown

The laboratory case for CBD in nausea comes largely from Linda Parker’s group in Canada, whose work is summarised in a 2011 review in the British Journal of Pharmacology. In rats and in shrews — the animals used because they can vomit, which rats cannot — CBD suppressed nausea-like behaviour, and it did so through the 5-HT1A serotonin receptor rather than through CB1.

The human evidence, and its limits

A Cochrane review of cannabinoids for chemotherapy-induced nausea and vomiting found them broadly comparable to older antiemetics and generally less well tolerated, with the trials mostly small, mostly old, and conducted with THC-containing medicines. The review does not tell you anything about CBD taken on its own, because those trials have not been run.

So the honest summary is: a plausible mechanism, a consistent animal literature with an awkward dose-response, a related compound that is more potent and harder to buy, and no randomised human trial of the thing actually being sold.

The complication nobody mentions

Cannabis can cause the symptom as well as settle it. Cannabinoid hyperemesis syndrome is a recognised condition in which long-term heavy use produces cyclical vomiting and severe abdominal pain, often with compulsive hot bathing, and it resolves on stopping. It is documented in the emergency medicine literature and is regularly mistaken for something else for months.

The cause changes what is worth trying

Anticipatory nausea, the real target

One part of this literature is more specific than the rest, and it is the part usually left out. The animal models did not only test sickness caused by a drug; they tested anticipatory nausea — the learned response in which the smell of a clinic, or the journey to it, produces the symptom before anything has been given.

That distinction matters because anticipatory nausea is the one form the standard antiemetics handle worst. It is a conditioned response rather than a chemical one, which is why the usual advice for it is to control the sickness completely from the first cycle so the association never forms, and why behavioural approaches appear in guidelines alongside drugs.

What a licensed antiemetic had to show

The bar this topic is usually measured against is worth stating plainly, because it is high and it has been cleared repeatedly by other compounds.

Ondansetron and the other 5-HT3 antagonists arrived in the late 1980s on randomised trials in chemotherapy patients that counted vomiting episodes directly rather than asking people how they felt. The NK1 antagonists that followed were added on trials showing they improved on that combination. Antiemetic guidelines now specify which combination goes with which chemotherapy regimen, at which doses, on which days — a level of detail that only exists because the trials were run.

There is one practical implication for anyone with nausea serious enough to matter. Effective antiemetics exist, they are prescribed by cause and severity, and the reason to raise persistent nausea with a doctor is not caution for its own sake — it is that the options are good and choosing between them needs the cause.

Reading a label without being taken in

A 2017 analysis in JAMA of 84 CBD extracts bought online found around seven in ten were inaccurately labelled, some with undeclared THC. On this topic that is not a small matter: THC is the cannabinoid with the licensed antiemetic, and it is also the one implicated in hyperemesis.

• Milligrams per serving, not the number on the front of the bottle.
• Spectrum: full spectrum carries a trace of THC, broad spectrum does not, isolate is CBD alone.
• A batch certificate matching the code on your container, showing what was found and what was looked for.
• Ginger, peppermint or B6 in the blend, each of which has its own evidence for nausea and any of which could be doing the work.