Factors affecting drug absorption and distribution

Drug absorption depends on the lipid solubility of the drug, its formulation and the route of administration. A drug needs to be lipid soluble to penetrate membranes unless there is an active transport system or it is so small that it can pass through the aqueous channels in the membrane. Typically drugs of the latter type are distributed into total body water. For weakly acidic and weakly basic drugs, absorption and distribution are pH dependent since only the unionized drug is lipid soluble. Strongly ionized drugs cannot cross membranes so they are not absorbed orally and cannot cross the blood-brain barrier. Traditional oral or parenteral administration may not achieve optimum therapeutic concentrations of drug at its site of action. After oral administration, metabolism of the drug in the gut or liver may result in low systemic bioavailability. Modern technology has produced delivery systems that allow precise control of drug input into the body by more unorthodox routes.