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What Influences How Bioavailable a Drug Is


When something is bioavailable, it will mean that it is absorbed by the body or by any living organism's system. If a drug is administered via an extra vascular route its absolute bioavailability will usually be less than one percent or you can also note it, for example, as F<100%. There are a great number of what is known as physiological factors that will influence the bioavailability of a drug (how a drug enters and is absorbed into systemic circulation). You must note that if a drug is taken along with food, the bioavailability will change. The consumption of multiple drugs simultaneously will influence the absorption of the drug you are looking at (other drugs greatly influence first-pass metabolism). The degree to which a drug is chemically degraded in addition to the speed of dissolution will be affected by intestinal motility. Diseases can also alter how a drug is absorbed.

Other factors to carefully consider (a doctor should know about all of these when prescribing drugs) when determining how bioavailable a drug is include, but are not limited to, the physical properties of the drug. This means the solubility, pKa and hydrophobicity are contributing factors. The formulation is also considered: the way the drug is manufactured; immediate release; modified release - which might refer to delayed, extended or sustained release. The formulation administering method should be carefully considered (this could either be in a fed or fasted state – in other words, whether or not the drug is taken with food).

Some more technical aspects of how bioavailable a drug is include the gastric emptying rate, the circadian differences, the overall health and well-being of the GI tract and, very importantly, the enzyme induction or inhibition by other elements: enzyme induction means a speeding up of the metabolism whereas enzyme inhibition means a decrease in the metabolic rate. The drug you are trying to determine the bioavalability of will react with drugs such as alcohol, nicotine and antacids, as well as foods like fruit juices and vegetables. Even age can affect how bioavailable a drug is, as well as the patient’s gender, diet and disease state.


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