The movement of a drug from the site of administration into the bloodstream is known as absorption.
Factors Influencing Drug Absorption
1. Physicochemical properties of the drug:
a. Physical state: A liquid form of the drug is better absorbed than solid formulations.
b. Lipid-soluble and unionized form of the drug is better absorbed than the water-soluble and ionized form.
c. Particle size: Drugs with smaller particle sizes are absorbed better than larger ones, e.g. microfine aspirin, digoxin, griseofulvin, etc. are well absorbed from the gut and produce better effects. Some of the anthelmintics have larger particle sizes. They are poorly absorbed through the gastrointestinal (GI) tract and hence produce a better effect on gut helminths.
d. Disintegration time: It is the time taken for the formulation (tablet or capsule) to break up into small particles and its variation may affect the bioavailability.
e. Dissolution time: It is the time taken for the particles to go into solution. Shorter the time,
better is the absorption.
f. Formulations: Pharmacologically inert substances like lactose, starch, calcium sulphate, gum, etc. are added to formulations as binding agents. These are not inert and may affect the absorption of drugs, e.g. calcium reduces the absorption of tetracyclines.
2. Route of drug administration: A drug administered by intravenous route bypasses the process of absorption, as it directly enters the circulation. Some drugs are highly polar compounds, ionize in solution and are not absorbed through the GI tract; hence are given parenterally, e.g. gentamicin. Drugs like insulin are administered parenterally because they are degraded in the GI tract on oral administration.
3. pH and ionization: Strongly acidic (heparin) and strongly basic (aminoglycosides) drugs usually remain ionized at all pH; hence they are poorly absorbed.
4. Food: The presence of food in the stomach can affect the absorption of some of the drugs. Food decreases the absorption of rifampicin, levodopa, etc.; hence they should be taken on an empty stomach for better effect. Milk and milk products decrease the absorption of tetracyclines. Fatty meal increases the absorption of griseofulvin.
5. Presence of other drugs: Concurrent administration of two or more drugs may affect their absorption, e.g. ascorbic acid increases the absorption of oral iron. Antacids reduce the absorption of tetracyclines.
6. Pharmacogenetic factors: Genetic factors may influence drug absorption. In pernicious anaemia, vitamin B12 is not absorbed from the gut due to a lack of intrinsic factors.
7. Area of the absorbing surface: Normally, drugs are better absorbed in the small intestine because of a larger surface area. Resection of the gut decreases absorption of drugs due to a reduced surface area.
8. Gastrointestinal and other diseases: In gastroenteritis, there is increased peristaltic movement that reduces drug absorption. In achlorhydria, absorption of iron from the gut is reduced. In congestive cardiac failure (CCF), there is GI mucosal oedema that reduces the absorption of drugs.