Factors Affecting Drug Absorption: Rotue Specific Factors
Degree of ionization
Only nonionized drug is absorbed. Ionized portion cannot cross biological membrane.
Basic drugs are largely nonionized in intestine.
Adicid drugs are largely nonionized in stomach.
However thick mucosa, mucous, small surface area, and shorter stay of drug in stomach limits the absorption from stomach.
So usually both types of drugs are mainly absorbed from intestine.
Sends the drug into the intestine where absorption is better → increases absorption.
gastric acid destroys penicillin G.
Enzyme peptidase destroys insulin.
Efflux transporter P-glycoprotein extrudes a fraction of absorbed drug back into the lumen
Decreased absorption of certain drugs e.g. digoxin and cyclosporine
Presence of Food
Usually decreases absorption because of following reasons
Dilutes the drug
Form complexes with drug that are poorly absorbed. e.g. calcium in milk forms complexes with tetracyclines.
Delays gastric emptying.
Absorption of certain drugs is increased. For example...
Fatty food increases absorption of lumefantrine and griseofulvin
Vitamin C keeps iron in ferrous form, which is absorbed better.
Hematinics containing iron → decreases absorption of tetracyclines
Liquid paraffin → decreases absorption of fat-soluble vitamins A, D, E & K.
First Pass Metabolism
Is degradation of drug after its absorption, before it reaches systemic circulation.
E.g., degradation of drug in liver (portal circulation) after its absorption from intestine.
Gastroenteritis → Decreases absorption of drugs.
Absorption of amoxicillin and pivampicillin is decreased.
Absorption of cephalexin is increased.
Subcutaneous & Intramuscular Route
Heat & Exercise
Increases blood flow
Abrasion on the surface → increases absorption.
Bioavailability in intravenous administration is always 100%.