When people talk about CBD, it is usually about milligrams. But the amount of CBD in a product does not tell the whole story of how much actually reaches the blood. That depends on bioavailability, which differs a lot depending on how CBD enters the body. In this article we explain what bioavailability is, why CBD is hard for the body to absorb and what studies show about different routes into the body.
This article is for information only and describes what research says about how CBD is absorbed in the body. It is not medical advice and does not give any advice on dosing or use. CBD can affect how some medicines are broken down in the liver, so if you take any medication, talk to a doctor before using CBD.
At a glance
- Bioavailability is the proportion of a substance that reaches the bloodstream unchanged.
- CBD is fat-soluble and dissolves poorly in water, which makes it hard for the body to absorb.
- When CBD is swallowed, much of it is broken down in the liver before it reaches the blood. Researchers estimate that only about 6 percent reaches the blood.
- A Swedish study from 1986 found that on average 31 percent of the CBD reached the blood when inhaled.
- The figures are based on few and often small studies and vary a lot between people.
What is bioavailability?
Bioavailability is a pharmacological term. It describes how much of a substance reaches the general bloodstream unchanged. A substance given directly into the blood, such as an injection into a vein, has 100 percent bioavailability by definition. Every other route into the body means that some of the substance is lost along the way.
How much is lost depends on the route the substance takes. Some routes bypass the digestive tract and the liver, while others pass through them and lose a large part of the substance before it reaches the blood.
Why is CBD hard for the body to absorb?
CBD is a highly fat-soluble molecule that dissolves poorly in water. The body’s fluids are largely water-based, so CBD has trouble dissolving and passing through the gut wall. This is one of the main reasons why absorption is low and varies so much.
The second reason is what is known as first-pass metabolism. What is absorbed from the gut first travels to the liver before it reaches the rest of the body. In the liver, CBD is broken down by enzymes, mainly CYP3A4 and CYP2C19. A large part of the CBD is therefore converted before it reaches the bloodstream. This is also why CBD can affect how some medicines are broken down, as the same enzymes are involved.
What studies show about different routes into the body
Inhalation
When CBD is inhaled, it is absorbed through the lungs and goes straight into the blood without passing through the liver first. A Swedish study from 1986 by Ohlsson and colleagues measured how much CBD reached the blood when volunteers smoked a cigarette containing labelled CBD and compared it with CBD given directly into a vein. On average, 31 percent reached the blood, but the variation between people was large. It is one of the few studies that has measured this, and it was done with smoking, not with today’s vaporizers.
By mouth
When CBD is swallowed, for example in a capsule, a food or an oil that is swallowed, it passes through the digestive tract and the liver. A 2020 review in the journal CNS Drugs describes that oral bioavailability in humans has been estimated at around 6 percent, with very large variation between people.
What is in the stomach at the same time also has a strong effect on absorption. In a 2018 clinical study of a medicine containing purified CBD, the total amount of CBD in the blood was about four times higher when participants had eaten a high-fat meal than when they were fasting. The fat helps the fat-soluble molecule dissolve and be absorbed in the gut, and this is one of the reasons why the figures in different studies differ so much.
Through the lining of the mouth
In theory, some CBD can be absorbed directly through the lining of the mouth and thereby bypass the liver. However, a 2018 systematic review in Frontiers in Pharmacology concluded that there are few studies, that they differ greatly in design and that much of what ends up in the mouth is swallowed anyway. How much is really absorbed through the lining of the mouth therefore remains unclear.
On the skin
The skin is built to keep substances out, and its outermost layer is an effective barrier. CBD used in creams and balms is therefore mostly absorbed locally in the skin and reaches the blood only to a small extent. Researchers are working on different formulations to increase absorption through the skin, but most studies have been done in animals. If you want to read more about products for the skin, you can read our article on the warming and cooling CBD muscle rub.
Why the figures vary so much
The percentages above should be read as rough guides, not exact values. Research on how CBD is absorbed in humans is still limited, and many studies are small. Much of the clinical research has also been done with medicines containing purified CBD, and the results cannot always be transferred to other products.
Absorption also differs between people, depending among other things on metabolism, body composition and what they have eaten. How the product is formulated also matters. Researchers are studying nanoemulsions and other formulations designed to make CBD easier for the body to absorb, but most of that research is still at an early stage.
What is in the product matters too
All research on bioavailability assumes that you know how much CBD the product contains. That is not always the case. When researchers tested 84 CBD products sold online in the US in 2017, only about 31 percent had a CBD content that matched the label. About 43 percent contained less than stated and 26 percent contained more.
That is why independent lab reports matter. A lab report shows how much CBD and other cannabinoids a product actually contains. We explain how to read such a report in our guide on CBD lab reports.
What does bioavailability mean?
Bioavailability is the proportion of a substance that reaches the bloodstream unchanged. A substance given directly into a vein has 100 percent bioavailability, while every other route into the body means some of it is lost.
What is the bioavailability of CBD?
It varies a lot. When taken by mouth it has been estimated at around 6 percent, and when inhaled a Swedish study found an average of 31 percent. The figures are based on few studies and differ a lot between people.
Why is bioavailability low when CBD is swallowed?
CBD is fat-soluble and dissolves poorly in water, which makes absorption in the gut difficult. What is absorbed also passes through the liver, where much of it is broken down by enzymes before it reaches the blood.
Does CBD from creams reach the blood?
Only to a small extent. The skin is an effective barrier, and CBD in creams and balms is mostly absorbed locally in the skin.
Can CBD affect medicines?
Yes, it can. CBD is broken down by the same liver enzymes as many medicines, including CYP3A4 and CYP2C19. If you take any medication, talk to a doctor before using CBD.
Sources
- Ohlsson, A. et al. (1986). Single-dose kinetics of deuterium-labelled cannabidiol in man after smoking and intravenous administration. Biomedical & Environmental Mass Spectrometry, 13, 77–83.
- Perucca, E. and Bialer, M. (2020). Critical aspects affecting cannabidiol oral bioavailability and metabolic elimination, and related clinical implications. CNS Drugs, 34, 795–800.
- Millar, S. A. et al. (2018). A systematic review on the pharmacokinetics of cannabidiol in humans. Frontiers in Pharmacology, 9, 1365.
- Taylor, L. et al. (2018). A phase I, randomized, double-blind, placebo-controlled, single ascending dose, multiple dose, and food effect trial of the safety, tolerability and pharmacokinetics of highly purified cannabidiol in healthy subjects. CNS Drugs, 32, 1053–1067.
- Bruni, N. et al. (2018). Cannabinoid delivery systems for pain and inflammation treatment. Molecules, 23, 2478.
- Bonn-Miller, M. O. et al. (2017). Labeling accuracy of cannabidiol extracts sold online. JAMA, 318, 1708–1709.
