Bioavailability is the proportion of a drug or other substance that enters the bloodstream when introduced into the body. This value varies due to many factors of the body. However, the most determinant factor is the way it is administered. There are four ways drugs can enter the human body: inhalation, ingestion, absorption, and injection.
Substances given intravenously have 100% bioavailability because they enter directly into the bloodstream. Substances that are taken orally have a much longer way to travel in order to enter the bloodstream. Therefore, they have a lower and variably bioavailability depending on how much the substance interacts with different acids or enzymes in the digestive system and liver (Flynn 2007).
Other methods of consumption also have variable bioavailability rates. Inhalation offers a very fast and efficient way to deliver some substances directly into the bloodstream through absorption in the lungs. However, this may have adverse and irritating effects on the lungs of some individuals(Abrams 2007). Sublingual absorption, while not as fast-acting as inhalation, enters into the bloodstream through glands under the tongue (Notcutt 2004). This can allow the substances to interact with enzymes in saliva prior to absorption or swallowing. Both of these possible occurrences could affect the substance’s bioavailability. Nasal sprays are another option for consumption, and allows a substance to be absorbed through the mucous membranes in the nasal passages. Transdermal patches can work to great effect for absorption directly through the skin (Lawrence 1997).
All of these methods offer a different path to the bloodstream and the absorption can vary from person to person. The substance in question also has an enormous effect on which method offers which level of bioavailability. In the case of CBD, the structure of the molecule has profound implications for its own bioavailability.
CBD products come in many forms. From topical ointments to edible candies, all with a fluctuating range of different concentrations. For example, 10mg of CBD taken with a vaporizer is not the same as 10mg of CBD taken as an edible. Again, substances are much more bioavailable through the lungs than through the digestive system. In CBD, this is partially due to the first pass metabolism effect (Pond 1984). Some of the CBD is metabolized by enzymes, mostly in the liver, but also in the digestive system before reaching the bloodstream. However, this isn’t the only factor leading to low oral bioavailability.
CBD is a lipophilic molecule, meaning that it is attracted to lipids, oils, and fats. This means that it does not dissolve well in water, if at all. The human digestive system is water based, but does has the ability to breakdown and absorb lipids as well. If CBD is not attached to some sort of fatty molecule, it has an extremely low absorption rate and most of it will simply pass through the digestive system without ever reaching the liver. This is the reason many oral CBD products have lipid based absorption enhancers or are made using easily digestible fats, such as oils found in foods (Cherniakov 2017).
CBD has been studied for its bioavailability in different delivery methods. Oral bioavailability ranges from about 6-15%, while vaporizing can be up to 50% (Ohlsson 1986)(Samara 1988). Topical and transdermal products have a rate somewhere around 45%, and nasal sprays around 40% (Paudel 2010). Each product has a unique path for entering the bloodstream and therefore has different availability.
Not all CBD products and their uses deliver the same results. A 20mg transdermal patch will deliver much more CBD than a 20mg oral capsule. A CBD vaporizer will take effect almost instantly, while an ointment may release the same amount of CBD over a much longer period of time. Also, individuals differ in which method is the most effective for their body and what their personal desired effect could be. It’s important to understand the differences between CBD products and how they’re absorbed in order to be able to find the right dose and treatment option for your body.