If a drug is required to act throughout the body or to reach an organ inaccessible to topical administration, it must be got into the blood and into other body compartments. Most drugs distribute widely, in part dissolved in body water, in part bound to plasma proteins, in part to tissues. Distribution is often uneven, for drugs may bind selectively to plasma or tissue proteins or be localised within particular organs. Clearly, the site of localisation of a drug is likely to influence its action, e.g. whether it crosses the blood-brain barrier to enter the brain; the extent (amount) and strength (tenacity) of protein or tissue binding (stored drug) will affect the time it spends in the body and thereby its duration of action.
Drug distribution, its quantification and its clinical implications are now discussed.
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