Processes occurring along with drug absorption when drug molecules travel down the gastrointestinal tract and the factors that affect drug absorption.
1. The gastrointestinal and other biologic membranes act like lipid barriers.
2. The un-ionized form of the acidic or basic drug is preferentially absorbed.
3. Most drugs are absorbed by passive diffusion.
4. The rate of drug absorption and amount of drug absorbed are related to the drug’s oil–water partition coefﬁcient (i.e., the more lipophilic the drugthe faster is its absorption).
5. Weak acidic and neutral drugs are absorbed from the stomach, but basic drugs are not.
For very weak acids, pK a values greater than 8.0 are predominantly un-ionized at all pH values between 1.0 and 8.0. Profound changes in the un-ionized fraction occur with pH for an acid with a pKa value that lies within the range of 2.0 to 8.0. Although the fraction un-ionized of even strong acids increases with hydrogen ion concentration, the absolute value remains low at most pH values shown.