How fast and how much of a drug is absorbed is affected by a number of factors which depend on the route of administration. The slower that a drug is absorbed the lower the peak concentration and the greater the chances that drug may be lost. The rate of absorption is a key determinant of a drug's bioavailability (see previous section).
The rate of absorption may be constant (zero order) or influenced by the drug concentration (first or second order).
Sites of drug administration and absorption:
(1) oral (gastrointestinal)
(2) transmucosal (oral, nasal, rectal, vaginal)
(3) transdermal
(4) intramuscular
(5) subcutaneous
(6) intravascular
(7) inhalation (transmucosal and alveolar)
(8) epidural or other CNS route
Factors affecting the rate of absorption:
(1) solubility of the drug at the pH of its surroundings
(2) precipitants in the environments (ions, proteins, etc)
(3) mobility of the environment (insufficient time for absorption)
(4) membrane barriers and the drug's lipid solubility
(5) physical barriers to absorption (fibrosis, edema, etc)
(6) surface area for absorption
(7) release of free drug from delivery vehicle
(8) chemical breakdown of the drug (metabolism by bacteria, enzymes, inflammatory cells, etc)
(9) perfusion of the tissue (affects how fast absorbed drug enters the circulation)
Factors impairing oral absorption of a drug:
(1) pH of gastric and small bowel fluids
(2) calcium, phosphate and ions in fluids
(3) food, blood and bacteria
(4) gut motility (vomiting, diarrhea)
(5) change in bowel wall (edema, fibrosis)
(6) decreased surface area (short bowel syndrome)
(7) rate of release from formulation
(8) poor perfusion (ischemia, hypotension, etc)
