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What Is Oral Bioavailability?

There are numerous studies that examine the oral bioavailability of different medications.
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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 21 November 2014
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Oral bioavailability has to be understood in context of the term bioavailability. This refers to the amount of medication that when entering the body actually gets into the circulation. Different factors affect amounts or ratios of available drug. When a medication is injected intravenously, for instance, it is usually completely used by the body because it goes straight into the bloodstream. This is not true when medications are taken by mouth, and oral bioavailability can be expressed as a ratio or percentage compared to the amount of chemical in the body when the medication is given in intravenous form.

It’s easy to understand that there are many things that can deplete, or possibly increase amount of available drug that is swallowed. The digestive system or the liver, in what is called first-pass metabolism, can absorb drugs in greater or less amounts. Depending on the degree to which this has affect on the medication, more or less of it ultimately becomes part of the bloodstream. Understanding oral bioavailability percentage can then be used to determine safe dosage, enough but not too much, and that is comparable to intravenous use.

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Each medication has to be viewed as highly individualized when it comes to oral bioavailability. Sometimes the way a medication is made and the form in which it is made may slightly change the rate and degree of absorption. For instance, it has been shown that certain brand name drugs are indeed superior to generics because of the way they’re made, even if they contain the same basic medicines. What makes them different is their formulations, extra ingredients, and how their chemical structure responds to first-pass metabolism.

There exists a plethora of studies on how individual medications are affected by oral bioavailability. This is also an incredibly rich area of research. When pharmacists and researchers design medicines, they must continually ask how design affects absorption. They must also consider what situations might make certain medications degrade and whether inactive ingredients, or the style of drug (liquid, caplet, capsule, time release) would have any effect on amount of medicine that hits the circulatory system.

Considering the above factors are not the only important matters. Oral bioavailability can change based on individual health and addition of other medications or substances. Those medications that are to be taken with food might be different when taken without food. Adding antacids to many common meds decreases oral bioavailability, and certain fruit juices or citrus fruits may increase or decrease percentage of drug that makes it to the circulatory system. These changes can be frightening since they might mean a person is inadequately medicated or over-medicated. Dosing has to take into account personal habits, health conditions, and especially the use of other drugs, whether these are over the counter, prescription or herbal.

Medications that have concerning changes when used with other things often receive warning labels, which essentially tell people oral bioavailability will change, and possibly in dangerous ways, if warnings are not observed. Many times changes are not hugely significant. Yet at all times, any differences in absorption could prove problematic and might interfere with successful health treatment.

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