Physico-chemical properties of drug:

) Physical state: Liquids are absorbed better than solids and crystalloids absorbed better
than colloids.
ii) Lipid or water solubility: Drugs in aqueous solution mix more readily than those in oily
solution. However at the cell surface, the lipid soluble drugs penetrate into the cell more
rapidly than the water soluble drugs.
iii) Ionization: Most of the drugs are organic compounds. Unlike inorganic compounds, the
organic drugs are not completely ionized in the fluid. Unionized component is
predominantly lipid soluble and is absorbed rapidly and an ionized is often water soluble
component which is absorbed poorly. Most of the drugs are weak acids or weak bases.
It may be assumed for all practical purposes, that the mucosal lining of the G.I.T is
impermeable to the ionized form of a weak organic acid or a weak organic base. These
drugs exist in two forms.

Acidic drugs: rapidly absorbed from the stomach e.g. salicylates and barbiturates.
Basic drugs: Not absorbed until they reach to the alkaline environment i.e. small
intestine when administered orally e.g. pethidine and ephedrine