Drug absorption from intestine
The ability of a drug to be absorbed is dependent on several factors of the drug: e.g. molecular size and shape, relative lipid solubility of the ionized and un-ionized forms, as well as the make-up of the intestinal wall.
For a drug to be absorbed, it should have a molecular weight of less than 1000 Daltons. Thus, a drug like vancomycin is not absorbed when given orally (see below).
The illustration shows how drugs with different properties are absorbed. The transport over the intestinal wall can be through the cells (transcellular transport) or through pores or tight junctions between cells (paracellular transport).
Lipophilic drugs are able to pass the fatty cellular membrane. This process is called passive transport or diffusion. Only unionised forms can cross the membrane by diffusion. Passive transport can go two ways, depending on the concentration gradient.
Hydrophilic drugs aren't able to cross the cellular membrane and use carrier-mediated transport via transporters, pores or carriers in the membrane. Transporters are often specific for their substrate(s). This way of transport is one-way and might cost energy and is then called active transport.
As the drug travels through the gastrointestinal tract, it moves through different pH environments. Since many drugs are either weak acids or weak bases, this results in a change in the drug's ionization status.
See how a change in the absorption will affect drug availability by clicking here and selecting oral availability (module 1).