Active Pharmaceutical Ingredient [API] « Back to Glossary IndexActive Pharmaceutical Ingredient Related Articles: Bioavailability and bioequivalenceBioavailability is the fraction (percentage) of an administered dose of unchanged drug that reaches the blood stream (systemic circulation). Bioavailability varies between different pharmaceutical ingredients and mode of administration.Glossary: BioequivalenceGlossary: Biotransformation« Back to Glossary IndexRelated Terms:Term: Active Pharmaceutical Ingredient