1) Passive diffusion Passive diffusion is used to describe the passage of (drug) molecules through a membrane which behaves inertly in that it does not actively participate in the process a Drugs absorbed according to this method are said to be passively absorbed.1) Passive diffusion ◼ Passive diffusion is used to describe the passage of (drug) molecules through a membrane which behaves inertly in that it does not actively participate in the process. ◼ Drugs absorbed according to this method are said to be passively absorbed