It includes various factors affecting drug absorption

1. ASIYABI.M
ASSISTANT PROFFESOR
DEPARTMENT OF
PHARMACOLOGY

2.  Stomach: Absorption of drug is very less in stomach
due to shorter gastric residence and less surface area.
Usually weak acidic drugs are absorbed through
stomach because they remain unionised .Drug like
antibiotic, proteins, and peptides are degraded due to
presence of low ph.
 Small intestine: Various gastic juice like bile acids, bile
salts and enzyme like maltase,lipase, are present in
small intestine. Every day around 10L of water is
absorbed by the small intestine.The increase in drug
absorption owing to the increase in water absorption.

3.  Large intestine: Large intestine has insignificant
absorption because it has small length and very short
mucosal surface area.It is mainly involved in the
absorption of water and electrolyte.
 Colon: It lacks well defined villi, hence decreased
absorptive surface area. When a drug is absorbed less
extent or degrades in the upper part of GI a portion of
drug moves in to the colon and gets absorbed.
PH OF GASTROINTESTINAL TRACT:
In any particular site of gastrointestinal tract unionised
drug has faster absorption than the ionised form,due
to greater lipid solubility.

4.  Absorption site: Particular drugs have specific
absorption sites at which sites at which they are
absorbed maximally.
 Protein binding: Those drugs that are bound to the
proteins present in membranes,blood or tissue show
difficulty in diffusion, absorption, reaching to the site
action, and interaction with receptor.
 Pre-systemic metabolism: Major metabolism site of
drug is liver. Pre-systemic drug metabolism occur in GI
tract or membrane. Absorbed dose before reaching the
blood circulation is reduced by biotransformation in
liver, and this is termed as first pass metabolism.This
happens to orally administered drug that are exposed
to the liver,after absorption.

6.  AGE: Absorption, distribution,metabolism, and
eliminated affect bioavailability of drug and it is
observed that these factor start reducing with
age.Whereas, absorption of drug in children is poor
due to incomplete development of systems.
 SEX: The relative proportion of muscular and adipose
tissue, also alter the absoption, distributionand
clearance of drug. Example pregnant famales may
reactdifferently to a drug.

7.  BODY WEIGHT: Lean body mass or fat mass both
affect drug distribution. Smaller volume and light
weight of body fluids have higher blood drug levels.
 ACTIVITY AND POSTURE: Acivity and posture
changesplasma volume or blood flow rate that brings
variation in drug pharmacokinetics of some drugs.
 FOOD: Usually in empty stomach drug absorption is
maximum. Rate of drug absorption changes in the
presence of food but not the extent of absorption.
Example: Aspirin, acetaminophen and rifampin that
few drugs have maximum absorption in the presence
of food.

8.  Medicines: Absorption of drug from stomach is almost
insignificant as compared to small intestine thus any
drug that affects the rate of gastric emptying can also
alter drug absorption. Example: anticholinergic drugs
inhibit or reduce gastric secretions and the antacids
usually elevate gastric Ph.
 DISEASE CONDITIONS: Diseases like GI ulcer, acidity,
achlorhydria, diarrhoes, constipation .decrease GI
secretions and GI transit. Analgesicslike aspirin and
paracetamol are absorbed well through small intestine.
Migraine delays gastric emptying.Resulting in delayed
absorption of these analgesics drug in case of migrain
attacks.