Absorption is the early phase of distribution when the drug enters the bloodstream from its site of administration. Two main parameters describe absorption: the ratio of absorbed dose over given dose and the rate of absorption. Both parameters influence the initial concentrations of the drug in blood, hence its concentration gradient and intensity of effects (when concentration dependent).

The main sites of absorption are the gut (oral route), connective tissues (subcutaneous route), muscles (intramuscular route), skin, eyes, nose, and various mucosa. These routes of administration are extravascular. The intravascular routes (intraarterial or intravenous) bypass the absorption phase as the drug is administered directly into the blood.

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