Molecular size
Small molecules are absorbed easier than larger ones. After intake, drugs have to cross different cell linings to reach the intended cells for the activation of the immune system. Therefore, small molecules will cross the walls faster as compared to larger ones due to permeability issues.
Dosage forms
The form of dosage affects the rate and extent of absorption. For instance, injected drugs are absorbed faster as compared to those given orally. Drugs in the form of tablets have to be broken down first before they can be absorbed. In the process, some parts may be partially broken down, and thus they cannot be absorbed. On the other side, drugs in liquid forms do not require any form of breakdown. Hence, they are absorbed immediately after administration.
Presence of other substances
Food or drugs interact with drugs to increase or reduce the rate of absorption. For instance, milk reduces the absorption of iron while Vitamin C enhances the same. Some substances enhance or inhibit the breakdown of certain drugs, which increases or reduced the rate of absorption. In addition, some substances may bind to drugs, thus inhibiting the absorption.
pH
Acidic drugs are absorbed well at low pH, while high pH enhances the absorption of basic drugs. Therefore, if acidic drugs are taken orally to pass through the stomach where pH is low, absorption will be inhibited or delayed.