Absorption of Drugs | factors affecting Drug Absorption

Absorption of Drugs 

  • Absorption of drugs is the transfer of a drug from its site of administration to the blood stream.
  • Once in the bloodstream, the drug can be transported to its site of action, metabolized, and eventually eliminated from the body.
  • The rate and efficacy of absorption depends on the route of administration, for Iv delivery absorption is complete, for example the total dose of the drug reaches the systemic circulation.
  • other routes may result in Only partial absorption, thus slower bioavailability.
  • Lipid soluble drugs willingly move over most biological membranes but water soluble drugs penetrate the cell membrane through aqueous channels.
  • Mechanism of absorption of drugs from the GIT include passive diffusion, facilitated diffusion, active transport, endocytosis and endocytosis.

Absorption of Drugs

Factors affecting drug Absorption:


Drugs with elevated lipid solubility are absorbed at a fast rate. Drugs specified in aqueous solution are more fastly absorbed than those given an oily solution, suspension or solid form.

Degree of ionization:

It is inversely proportional to absorption; greater the degree of ionization, Lesser the absorption. Lipid soluble drugs are in a unionized form and are readily absorbed where as water soluble drugs are in ionized form and can be absorbed if they have very small molecular size.

Pharmaceutical preparations:

Solutions are better absorbed than suspensions, smaller the molecular size of powders, the more efficient their absorption.

Drugs that are formulated as sustained-release or enteric-coated tablets may have delayed or altered absorption patterns compared to immediate-release tablets.

Route of administration:

The route of administration refers to the way in which a drug is introduced into the body. Different routes of administration have different rates and degrees of drug absorption.

For example, drugs administered intravenously (IV) have 100% bioavailability, meaning the entire dose reaches the bloodstream. On the other hand, oral drugs may undergo first-pass metabolism in the liver before reaching the bloodstream, resulting in reduced bioavailability.

Absorption from parenteral route is rapid than oral route and absorption from intramuscular route is rapid than from subcutaneous route.

Surface area, concentration of drug and local blood flow these are directly proportional to absorption of the drug.

Gastric Emptying:

The rate of gastric emptying can affect the absorption of orally administered drugs. Drugs that are absorbed in the small intestine may be affected by the rate at which they are emptied from the stomach.

Expression of P-glycoprotein:

P-glycoprotein is a multidrug transmembrane transporter protein responsible for transporting various molecules (including drugs) across cell membrane. it is expressed throughout the body.

Functions of P-glycoprotein:

Liver -> Transports the drugs to bile for elimination.

Kidney -> Pumps drugs into urine for excretion.

Placenta -> transport drugs back into maternal blood thereby reducing foetal exposure to drugs.

Brain capillaries -> Pumps drugs back into blood thereby limiting drug access to brain.

Intestine -> Transport drugs into the intestine lumen and reducing drug absorption into the blood.

Physicochemical Properties of the Drug:

The physicochemical properties of a drug can affect its solubility, permeability, and stability, which in turn affect drug absorption. For example, drugs that are lipophilic (fat-soluble) tend to be absorbed more readily than drugs that are hydrophilic (water-soluble).

GI pH:

The pH of the gastrointestinal (GI) tract can affect the solubility and stability of drugs, which in turn can affect their absorption. For example, some drugs may be absorbed more readily in an acidic environment, while others may require an alkaline environment.

Other factors:

• If a drug moves through the GIT very fast as seen with severe diarrhoea is not good absorbed; conversely anything that slow down the transport of drugs from stomach to intestine, slow the rate of absorption.

• Presence of food and other substances in the GIT may interfere with the absorption of drugs Some drugs may be absorbed more readily when taken with food, while others may be absorbed more slowly or incompletely. for example tetracyclines are antibacterial agents (protein synthesis inhibitors) that are poorly absorbed if given with calcium.

• Exercise decreases the absorption of drugs taken by mouth because blood is diverted away from the GIT to skeletal muscles.

The absorption of drugs can occur through various routes of administration, including:

Oral Administration:

Oral administration is the most common route of drug administration. Drugs taken orally must first pass through the gastrointestinal tract, where they may undergo metabolism and absorption into the bloodstream.


Injection is a method of administering drugs directly into the bloodstream or into a specific tissue or organ. Injection can be performed intravenously, intramuscularly, subcutaneously, or intra-arterially.


Inhalation is a route of administration that involves inhaling drugs in the form of a gas, vapour, or aerosol. The lungs have a large surface area for absorption, allowing drugs to be absorbed quickly and efficiently.

Topical Application:

Topical application involves applying drugs directly to the skin, mucous membranes, or other body surfaces. Topically applied drugs can be absorbed through the skin or other tissues and into the bloodstream.

Learn more

What is absorption of drugs?

What is drug absorption?

Factors affecting drug absorption

Factors modifying drug absorption

Top 5 factors affecting drug Absorption

Affect of solubility on drug absorption

affect of pharmaceutical preparations on absorption of drugs

Affect of route of administration on absorption of drugs

Effect of GI tract pH on absorption of drugs

Affect of physiochemical properties of drugs on absorption of drugs

Effect of the P-glycoprotein on absorption of drugs

Functions of P-glycoprotein

Effect of gastric emptying on absorption of drugs

Post a Comment