The gastrointestinal absorption of most drugs from solution occurs in this manner in accordance with first order kinetics in which the rate is dependent on drug concentration The magnitude of the permeability constant, depends on a the diffusion coefficient of the drug a the thickness and area of the absorbing membrane a the permeability of the membrane to the particular drug.The gastrointestinal absorption of most drugs from solution occurs in this manner in accordance with first order kinetics in which the rate is dependent on drug concentration. The magnitude of the permeability constant, depends on ◼ the diffusion coefficient of the drug ◼ the thickness and area of the absorbing membrane ◼ the permeability of the membrane to the particular drug