Definition of Bioavailability

Bioavailability is the amount of a drug’s active ingredient the body absorbs and the length of time it takes for that ingredient to cause an effect in the body. A common means of determining bioavailability is to measure the concentrations of the drug in the blood circulation or in the urine at certain time intervals. Doctors know the spectrum of bioavailability and calculate dosage to obtain the desired therapeutic concentration of the drug.

For most drugs the spectrum of activity provides adequate therapeutic levels and tests to measure the drug’s concentrations are not necessary. Narrow therapeutic index (nti) drugs such as the anticoagulant warfarin, the antiarrhythmic digoxin, and hormone supplements such as levothyroxine (thyroid hormone) require diligent assessment and monitoring because the margin between therapeutic and toxic is very close. The doctor may also assess the drug’s bioavailability through observation of clinical changes, such as an infection that improves with antibiotic therapy or blood pressure that drops with antihypertensive medications.

For the most part pharmaceutically equivalent drugs (generic drugs) have consistent bioavailability across manufacturers and are interchangeable from this perspective. The exceptions are NTI drugs, for which doctors and pharmacists recommend staying with the same brand name of drug for the duration of treatment. Which brand does not matter so much as that the brand remains consistent.

This is because even minute variations in the manufacturing process, as is inherent in different formulations of the same drug product, affect the way the body absorbs and metabolizes the drug. Other factors that influence bioavailability are interactions with foods, other drugs, and medicinal herbs and botanicals. Health conditions such as gastrointestinal malabsorption, renal failure, or liver failure, as well as the person’s age and weight, and metabolic disorders, also affect bioavailability.


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