DEFINITION of 'Bioavailability'

The speed and potency at which a drug enters the bloodstream. Bioavailability is affected by many things, from the way a drug is designed and produced to a patient’s genetic makeup. A drug’s bioavailability is a key factor in how well it helps a patient and in whether it causes unwanted side effects.

BREAKING DOWN 'Bioavailability'

Two drugs with the same amounts of the same active ingredients will not necessarily have the same bioavailability because of differences in the drugs’ inactive ingredients. This possible difference is one reason why generic drugs must be tested and approved before they can be marketed as name-brand equivalents. If two drugs with the same active ingredients do have the same bioavailability, they are said to be bioequivalent. If the same patient takes both drugs, they should not be able to tell the difference in the way each drug affects their body. If a drug company manufacturing a generic is having trouble creating a drug with the same bioavailability as the original, the company’s profits and stock price may suffer.

Drugs have different levels of bioavailability. Drugs taken by mouth generally have low bioavailability because they have to pass through the intestinal wall and the liver before reaching the bloodstream. A patient’s age, gender, health conditions, surgical history, stress levels, diet, physical activity levels and other drugs can all increase or decrease bioavailability. In addition, a drug can be taken many ways: as a pill, injection, patch, inhaler, eye drop, nasal spray and more. Each route of administration creates different potential levels of drug bioavailability in the human body. Drugs injected into a vein are 100% bioavailable since they are delivered directly to the bloodstream; drugs taken by any other method are less than 100% bioavailable because they have to pass through some other part of the body before entering the bloodstream. The technical term scientists use to describe bioavailability is “area under the plasma-concentration time curve.”

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