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How does bioavailability affect whether a medicine works in the body?

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A lab technician wearing protective gloves and face shield lifts a rack of deep-frozen cells from a cryopreservation tank emitting vapor in a laboratory setting. — pharmaceutical industry A technician withdrawing deep-frozen cells for in vitro culture at a laboratory of a German pharmaceutical company.

Bioavailability is a critical factor in determining whether a medicine is effective, as it refers to the degree and rate at which a drug is absorbed into a living system and made available at the site of physiological activity. For a medication to produce a therapeutic effect, it must reach its target site in an adequate concentration.12

If a drug has low bioavailability, it may not reach the necessary levels in the bloodstream to be effective, rendering the treatment unsuccessful. Conversely, if bioavailability is too high or if the drug is not metabolized or excreted as expected, it can lead to toxic levels in the body. Because individuals vary in their ability to absorb, metabolize, and excrete substances—influenced by factors such as age, genetics, and existing health conditions—the pharmaceutical industry conducts pharmacokinetic studies to determine the appropriate dosage and frequency required to maintain a drug within its therapeutic window. This window represents the concentration range where the drug is both safe and effective; falling below this range results in an ineffective treatment, while exceeding it risks toxicity.34