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Oral bioavailability refers to the amount of drug that enters the circulation when ingested. Factors such as first-pass metabolism, drug formulation, and individual health can affect oral bioavailability. Understanding it is crucial for determining safe dosages and avoiding dangerous interactions with other drugs or substances.
Oral bioavailability must be understood in the context of the term bioavailability. This refers to the amount of drug that actually enters the circulation when it enters the body. Several factors affect the amounts or ratios of available drug. When a drug is injected intravenously, for example, it is usually completely used up by the body because it goes directly into the bloodstream. This is not true when drugs are taken orally, and oral bioavailability can be expressed as a ratio or percentage to the amount of the chemical in the body when the drug is given intravenously.
It’s easy to see that there are many things that can deplete, or perhaps increase, the amount of available drug that is ingested. The digestive system or liver, in what is called first-pass metabolism, can absorb drugs in larger or smaller quantities. Depending on the degree to which this affects the drug, more or less of it eventually becomes part of the bloodstream. Understanding the percent oral bioavailability can then be used to determine a safe dosage that is sufficient but not excessive, and that is comparable to intravenous use.
Each drug must be considered highly individualized when it comes to oral bioavailability. Sometimes the way a drug is made and the form in which it is made can slightly change the rate and extent of absorption. For example, some brand-name drugs have been shown to actually be superior to generics in the way they are made, even though they contain the same basic drugs. What makes them different are their formulations, extra ingredients, and how their chemical structure responds to first pass metabolism.
There is a plethora of studies on how individual drugs are affected by oral bioavailability. This is also an incredibly rich area of research. When pharmacists and researchers design drugs, they must continually ask how the design affects absorption. They also need to consider what situations might cause some drugs to degrade and whether the inactive ingredients or style of drug (liquid, capsule, capsule, timed release) would have any effect on the amount of drug affecting the circulatory system.
Considering the above factors are not the only important issues. Oral bioavailability can change based on individual health and the addition of other drugs or substances. Medications that should be taken with food may be different when taken without food. The addition of antacids to many common drugs reduces oral bioavailability, and some fruit or citrus juices can increase or decrease the percentage of drug reaching the circulatory system. These changes can be scary as they could mean that a person is under-medicated or over-medicated. The dosage should take into account personal habits, health conditions and especially the use of other drugs, whether over the counter, prescription or herbal.
Drugs that have concerning changes when used with other things often receive warning labels, which essentially tell people that oral bioavailability will change, and possibly in dangerous ways, if the warnings are not followed. Many times the changes aren’t terribly significant. However, at any given time, any difference in absorption could prove problematic and could interfere with successful healthcare treatment.
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