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What is Bioavailability?

Tricia Christensen
Updated May 21, 2024
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Bioavailability is a term used by several branches of scientific study to describe the way chemicals are absorbed by humans and other animals. Bioavailability was once strictly ascribed to pharmacology, but now has broad applications in environmental sciences as well. Examining a substance’s bioavailability in pharmacological studies helps to determine dosages of particular medications. A bioavailability measurement of a medication, when it reaches circulation in the body, describes aspects like absorbency and half-life. It can evaluate medication delivery as well.

Intravenous administrations of medications are considered to have 100% bioavailability because they do not pass through the stomach. They are immediately in the circulatory system. However, other medications administered at the same time may reduce the effects of an intravenous administration and affect its bioavailability.

Pharmacology also distinguishes between absolute bioavailability and relative bioavailability. Absolute bioavailability is the measurement of a medication once it passes through the gut and is released into the circulatory system. Relative bioavailability is a term used to compare different formulations of the same medication, for example brand name versus generic.

Some studies have found that certain generic preparations are not equivalent in bioavailability to brand name versions of medications. One example of this is the drug Synthroid, which is usually marketed in generic form as thyroxidine. Many patients who use thyroid replacement therapy find that thyroxidine is not as effective as Synthroid. Technically, the two medications should be equivalent, but differences in the bioavailability of the two forms have been noted in studies. This does not, however, change most insurance coverage in regards to this medication. Generally, one will still have to pay extra to purchase Synthroid.

Other factors can affect bioavailability when drugs are taken orally. Slow or quick metabolism can change the absorption rate of medication. Factors outside of the body, like improper storage of medications, can lead to less than expected bioavailability results.

Bioavailability, when used in environmental sciences, evaluates the rate and amount of toxic substances that may occur in the body. One example of this is the recent concern over mercury levels in fish. Some fish contain high levels of mercury, a poison, which can lead to severe illness when ingested in high quantities. People who eat a lot of fish may be subject to mercury poisoning. Recent recommendations by the US FDA suggest consuming mercury-high fish no more than once a week.

Another bioavailability study that has raised concern about our relationship to the environment is the discovery of small amounts of jet fuel secreted in human breast milk. At this point, no one has suggested that mothers stop breastfeeding their children, but the study raises concerns about many things we take for granted, like flying planes or rockets, which may in fact increase risk of health problems to children and adults.

Bioavailability studies may also compare animal or plant populations to the presence of toxic chemicals in an environment. Of concern is the degree to which these chemicals are present in animals when they reach the circulatory system, and what possible effects this may have on animal populations.

For instance, animal populations, which were later consumed by humans, consumed plant populations covered with DDT. Some effects in humans were increases in certain cancers and autoimmune disorders. Humans could also consume DDT through plant material, creating similar issues. These studies resulted in bans on DDT, which was once commonly used as a pesticide.

The bioavailability of toxins in our environment, when studied, may make significant impacts on our life quality and choices. These studies are invaluable, as they can help us approach our ecology with a sense of responsibility and a view to the future.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By anon42256 — On Aug 20, 2009

what is the most bioavailable drug?

By anon28688 — On Mar 20, 2009

Doctor ordered opana ER 10mg 2 x's/day I was and still am ordered vicodin 7.5/750 and oxycodone as needed. Can either one of these be given together at the same time? If not how much time should be spaced between them?

By anon19673 — On Oct 17, 2008

I am a chronic pain patient and am on narcotics as part of my pain management program. One of the web sites I visit had a discussion on Opana ER vs Oxycodone ER in relation to strength. The question that was brought up is that even though Opana is supposed to be a more potent med than the Oxycodone many people are saying they are not getting the relief they expected from The Opana. One of the posters on the site claims it is due to the bioavailability of Opana being 10% and that of Oxycodone is 90%. Is this correct? I've been trying to get a good understanding of bioavailability for some time. I have included one of the posts from the site as the example. Thank you for your time. Dennis

By Kartik — On Sep 04, 2008

I was reading some literature related to drug delivery in the eye and came across the term bioavailability. From what I managed to understand, its better for ocular drugs to have LOW bioavailability, because they need to end up inside the eye rather than in the blood stream. Am I correct in my assessment?

By anon11280 — On Apr 12, 2008

Hi, I became aware of this situation when I took this a chemical dependency counseling course at the u of minn, i had had a gastric bypass in 1980 so became VERY concerned on how this works when parts of the stomach are removed. Recently I have been so tired and finally got put on the generic of Synthroid as you stated, and i still feel lousy but doc says my blood levels are at low normal so no increases. I am afraid that docs are not getting taught about this since i have not heard the word since my class in 1982. All i hear is the generic drug is the same as the brand, and get scared for people. Just a note, and now that my bypass has come partly undone, and i get hyperinsulemia very easily, what now. Thank you.

By anon674 — On May 01, 2007

Bioavailabilty is commoly related to absorption and excretion. Why this term is not related to altered intake or altered usages by cells? Glucose's can be absorbed more and excreted more. But its effectiveness can also be effected due to decreased uptake by cells due to insulin resistance. As such, how such resistance by cells of any excess/accumulated substance is covered by bio-availabilty? Further, substances usages may also effected due to changes in blood flow by changes vascular tone or be changed vascular preambabilties. How these factors are covered by bio-abailabilty?

Best wishes.

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia...
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