Factors Affecting Drug Absorption: General Factors
These are the general factors that affect drug absorption by all routes.
Aqueous Solubility of the Drug
The drug must dissolve in aqueous biophase in order to be able to cross the biological membrane.
The aqueous solubility depends on:
Disintegration: breaking down of solid dosage form into fine particles.
Dissolution: dissolution of active drug molecules from fine particles into the solution.
The more the aqueous solubility → the faster the absorption. e.g.:
Tablets take time to disintegrate and dissolve → absorption from them is slower.
In syrup, the drug is already dissolved and ready to be absorbed → absorption from them is faster.
Concentration of the Drug
Diffusion of drug molecules is driven by concentration gradient
↓
The higher the concentration of drug
↓
The faster the absorption.
Area of Absorbing Surface
The more the absorbing surface area
↓
the more the absorption.
Blood Flow at the Absorbing Surface
Blood flow removes the absorbed drug from the site of absorption
↓
concentration gradient is maintained for absorption of more drugs
↓
so the more the blood flow
↓
the faster the absorption.