Bioavailability is the share of a dose that reaches your bloodstream in active form. For CBD it is low. When CBD is swallowed on an empty stomach, only about 6% gets through.1 The route you use and whether you have eaten change that figure a great deal, and several popular claims about absorption have never been properly measured.
Why is so little swallowed CBD absorbed?
The main reason is the liver. Blood from the gut passes through the liver before it reaches the rest of the body, and the liver breaks down much of the CBD on the way. This is called first-pass metabolism. Absorption from the gut is also erratic.1 The result is a low and variable figure.
How do the different routes compare?
| Route | Bioavailability in people | How fast it peaks | How good is the measurement? |
|---|---|---|---|
| Swallowed (oil, capsule, oral solution), empty stomach | About 6%1 | About 2.5 to 5 hours for the oral solution with regular dosing2 | Measured |
| Swallowed with a high-fat meal | Total exposure about 4 times the fasted level2 | As above | Measured in several studies |
| Inhaled (smoked) | About 31%, range roughly 11 to 45%3 | 5 to 10 minutes for inhaled aerosol1 | Measured |
| Under the tongue or mouth spray | Estimates of about 12 to 35% in one review4 | Not well established | No well-measured absolute figure in humans |
| Creams and balms | Low absorption into the blood5 | Not applicable | Limited |
| Transdermal patches and gels | Reach the blood to a limited degree5 | Not well established | Limited |
A 2018 systematic review of human studies concluded that, apart from the oral and smoked figures, no other route has a properly measured absolute bioavailability in humans.3 “Absolute” bioavailability means measured against a dose injected into a vein, which is the 100% reference point.
Is under the tongue really better than swallowing?
This is the most common absorption claim, and the evidence for it is weak. The idea is that CBD held under the tongue passes straight into the blood through the lining of the mouth and avoids the liver. In practice, much of a sublingual dose is swallowed. Estimates in one review ranged from about 12% to 35%, and there is no well-measured absolute figure in humans.4
By contrast, the effect of food on swallowed CBD has been measured repeatedly and is large.26 If the question is what reliably changes how much CBD reaches the blood, food has far better evidence than the under-the-tongue technique. See taking CBD with food.
How much does food change absorption?
A high-fat, high-calorie meal raised the peak CBD blood level about 5-fold and total exposure about 4-fold compared with fasting. A low-fat meal raised them about 4-fold and 3-fold. These figures come from 750 mg and 1,500 mg doses of the pharmaceutical oral solution.2 A separate study of a single 1,500 mg dose in healthy adults found a 4.2-fold rise in total exposure with a high-fat meal.6 A 2024 pooled analysis also linked taking CBD with food to higher bioavailability.7
Does higher bioavailability mean a better product?
Not necessarily. Three points are worth keeping in mind.
- The evidence was built on swallowed CBD. The trials behind the registered epilepsy medicine used an oral solution, swallowed, at 10 to 25 mg per kilogram of body weight per day.2 Its low bioavailability is already built into those doses.
- More absorbed means more of everything. A higher blood level raises the chance of dose-related side effects as well as any benefit. The TGA’s assessment of the registered CBD medicine identified higher dose as a risk factor for liver abnormalities.8
- Route changes the rules. The Australian pharmacist-only (Schedule 3) category covers oral products only. Vapes and topical products are excluded.9 See CBD vapes and CBD flower and CBD creams and topicals.
Be cautious with marketing that quotes a precise bioavailability percentage for a particular oil or delivery method. For most routes and formulations, the human studies needed to support such a number have not been done.3
Why do two people absorb the same dose differently?
Absorption of swallowed CBD is described as erratic and variable.1 Several things add to the spread:
- Meals. Fasted versus fed makes a several-fold difference.2
- Other medicines. CBD is broken down by the liver enzymes CYP2C19 and CYP3A4. Strong enzyme-inducing medicines such as rifampicin lowered CBD exposure by 32%.2 See CBD drug interactions.
- Alcohol. Taking CBD with alcohol raised total CBD exposure by 63%.2
- Liver function. The US prescribing information sets lower doses for people with moderate or severe liver impairment.2
Does what is in the bottle match the label?
Bioavailability only matters if the dose going in is what you think it is. In a US study of 84 CBD products bought online, only 26 (about 31%) contained an amount of CBD within 10% of the label.10 In Australia, the TGA laboratory tested 22 high-volume medicinal cannabis products supplied on prescription, and results were mostly close to the label.11 See is what is on the label in the bottle?.
What does this mean for dosing?
It explains two things that often confuse people.
First, it is part of why doses in trials look so large. If only a small share of a swallowed dose reaches the blood, the dose swallowed has to be big to achieve a given blood level. The CBD dosage guide lists what trials used.
Second, it is why a dose in milligrams does not carry over from one route or routine to another. The same number of milligrams taken fasted, taken with a meal or inhaled leads to different blood levels. Any change in how you take a prescribed product is worth raising with your prescriber first.
Footnotes
-
World Health Organization, Expert Committee on Drug Dependence. Cannabidiol (CBD) critical review report, 2018. https://cdn.who.int/media/docs/default-source/controlled-substances/whocbdreportmay2018-2.pdf ↩ ↩2 ↩3 ↩4 ↩5
-
US National Library of Medicine, DailyMed. Epidiolex (cannabidiol) oral solution: prescribing information, revised 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8bf27097-4870-43fb-94f0-f3d0871d1eec ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9
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Millar SA, et al. A systematic review on the pharmacokinetics of cannabidiol in humans, 2018. https://pubmed.ncbi.nlm.nih.gov/30534073/ ↩ ↩2 ↩3
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Review of cannabidiol delivery routes and oromucosal bioavailability estimates (journal article). https://pmc.ncbi.nlm.nih.gov/articles/PMC7558665/ ↩ ↩2
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Review of topical and transdermal delivery of cannabidiol (journal article). https://pmc.ncbi.nlm.nih.gov/articles/PMC11172078/ ↩ ↩2
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Taylor L, et al. A phase I, randomized, double-blind, placebo-controlled, single ascending dose, multiple dose, and food effect trial of highly purified cannabidiol in healthy subjects. CNS Drugs, 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC6223703 ↩ ↩2
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Meta-regression of factors affecting cannabidiol bioavailability, 2024 (journal article). https://pmc.ncbi.nlm.nih.gov/articles/PMC11397906/ ↩
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Therapeutic Goods Administration. Australian Public Assessment Report for cannabidiol (Epidyolex). https://www.tga.gov.au/sites/default/files/auspar-cannabidiol-210115.pdf ↩
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Retail Pharmacy magazine. TGA announces OTC access for low-dose CBD (news report of the TGA decision). https://retailpharmacymagazine.com.au/tga-announces-otc-access-for-low-dose-cbd/ ↩
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Bonn-Miller MO, et al. Labeling accuracy of cannabidiol extracts sold online. JAMA, 2017. https://jamanetwork.com/journals/jama/fullarticle/2661569 ↩
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Therapeutic Goods Administration. Testing of medicinal cannabis products being supplied through SAS in Australia. https://www.tga.gov.au/resources/publication/tga-laboratory-testing-reports/testing-medicinal-cannabis-products-being-supplied-sas-australia ↩
Common questions
What is the bioavailability of CBD oil?
When CBD is swallowed on an empty stomach, about 6% of the dose reaches the bloodstream, according to the World Health Organization's review. Absorption is erratic and varies between people. Taking it with food raises the amount absorbed several-fold.
Is CBD under the tongue absorbed better than swallowed CBD?
It is often claimed, but not proven. One review quoted estimates of about 12 to 35%, which is a wide range, and noted that much of a sublingual dose is swallowed anyway. No well-measured absolute figure exists in humans.
Which way of taking CBD has the highest bioavailability?
Of the routes that have been properly measured in people, inhalation is highest at about 31%, with a range of roughly 11 to 45%. That does not make it the best choice. Inhaled products fall outside the pharmacist-only category in Australia and the strongest clinical evidence is for swallowed CBD.
Do CBD creams get into the bloodstream?
Very little does. Creams and balms have low systemic absorption and are used for a local effect on the skin. Transdermal patches are engineered differently and reach the bloodstream to a limited degree.
This page is general information, not medical or legal advice. Laws and evidence change; we show the date we last checked the facts at the top of every page. Talk to your doctor or pharmacist before starting, stopping or changing any medicine.
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