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Factors Affecting Drug Absorption: Rotue Specific Factors



Oral Route

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.

Gastric Emptying

  • Sends the drug into the intestine where absorption is better → increases absorption.

Degradation

  • Decreases absorption.

  • Example:

    • gastric acid destroys penicillin G.

    • Enzyme peptidase destroys insulin.

Extrusion

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

    1. Dilutes the drug

    2. Form complexes with drug that are poorly absorbed. e.g. calcium in milk forms complexes with tetracyclines.

    3. Delays gastric emptying.

  • Absorption of certain drugs is increased. For example...

    1. Fatty food increases absorption of lumefantrine and griseofulvin

    2. Vitamin C keeps iron in ferrous form, which is absorbed better.

Concurrent Drugs

  • 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.

  • Decreases bioavailability of certain drugs e.g. levodopa, nitroglycerine, propranolol etc.

Gastrointestinal Diseases

  • Gastroenteritis → Decreases absorption of drugs.

  • Coeliac disease

    • Absorption of amoxicillin and pivampicillin is decreased.

    • Absorption of cephalexin is increased.


Subcutaneous & Intramuscular Route

Heat & Exercise

↓

Increases blood flow

↓

increases absorption


Topical Route

  • Abrasion on the surface → increases absorption.


Intravenous Route

  • Bioavailability in intravenous administration is always 100%.

 

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