Bio-availability refers to the proportion of a drug reaching the systemic circulation after a particular route of administration. The most important factor is ﬁrst-pass metabolism – that is, pre-systemic metabolism in either the intestine or the liver. Many lipid-soluble drugs such as beta blockers (see BETAADRENOCEPTOR-BLOCKING DRUGS), some tricyclic ANTIDEPRESSANT DRUGS, and various opiate ANALGESICS are severely aﬀected. Food may aﬀect bio-availability by modifying gastric emptying, thus slowing drug absorption. Ingested calcium may combine with drugs such as tetracyclines, further reducing their absorption.
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