Drug absorption


Definition(Drug absorption):Drug molecules cross the cell membrane to enters into the body(or systemic circulation) is called drug absorption.

Site of absorption:

  1. Alimentary tract:
  • Buccal cavity.
  • Stomach
  • Rectum
  • Intestine
  1. Parentral site.
  2. skin
  3. Respiratory.

 Process of drug absorption:

[A]Simple transport/passive transfer:

  • Simple diffusion.
  • Filtration

[B]Specialized transport:

  • Active transport.
  • Facilitaed transport.
  • Endocytosis:Phagocytosis,Pinocytosis.
  • Ion transport.
  • Ion-paired transport:Vit:A,D,E,K anticancer drug.

[A]Simple transport/Passive transfer:

Simple diffusion: Transfer of drug from one compartment to another compartment along the conc,electrochemical or pressure gradient without energy & carrier is called simple diffusion.


  • Commonest phenomenon in drug adsorption.
  • Energy is not needed.
  • Carrier is not needed.
  • Bi-directional.
  • Slow process.
  • It occurs along the cone,electro-chemical or pressure gradient.
  • No selective & saturability.
  • 70% drugs are transported by this process.
  • Example:Aspirin,Morphine,Paracetamol etc.

Filtaration:It is a process by which fluid is filtered from the through a filtering memberane by a filtering pressure.


  • It requires no energy.
  • It is independent of lipid solubility of drugs.
  • It depends on:Down hill gradient,Filtration pressure.
  • Example: Excretion through glomerular membrane-urea,ethyl glycol.

[B] Specialized transport:       

Active transport:An energy requiring carrier mediated uphill transport process is called active transport.


  • Neuronal membrane.
  • Choroid plexus.
  • Hepatocytes
  • Renal tubular cells.


  • Very rapid process.
  • Carrier is required.
  • Competitive in nature.
  • Exhibits selectivity & saturability.
  • Up hill transport process.
  • Less common.
  • Example:Levo-dopa,Streptomycine.

Facilitated diffusion:The diffusion which is facilitated by carrier is called facilitaed diffusion.


  • Carrier is required.
  • It has got selectivity & saturability.
  • It requires no energy.
  • Speed is medium.
  • Down hill transport process.
  • Example:Tetracycline,Pyrimidine.

Difference between Simple & Facilitate diffusion:

Simple diffusion Facilitate diffusion
1.Carrier is not needed. 1.Carrier is required.
2.Slow process. 2.Speed is medium.
3.No selectivity & saturability. 3.It has got selective & saturability.
4.It depends on lipid/water partition co-efficient. 4.It is independent of lipid/water partition co-efficient.

Difference between Simple & Active transport:

Simple diffusion Active transport
1.Diffusion showa first order kinetics. 1.Transfer against concentration gradient.
2.Cellular energy is not required. 2.This process expend cellular energy.
3.It depends on size of particles. 3.More rapid transfer than by simple diffusion.


 Factor modifying drug absoption:

Lipid solubility:

  • More lipid soluble drug→More absorption.
  • Less lipid soluble drug→Less absorption.



  • Molecular weight:Drugs of low M.W→10-1000.↓M.W→↑Rate absorption.


  • Size of the drug:Lipid soluble drug or any size can be easily absorbed.
  • Small size→More absorption.
  • Larger→Less absorption.


  • Degree of ionization:
  • More ionization→More water solubility→Less absorption.
  • Less ionization→Less water solubility→More absorption.


  • Formulation of drug:
  • Gaseous drugs are better absorbed than liquid drug.
  • Liquid drugs are better absorbed than soild drug.


  • pH of the environment:Acidic drug in acid media & alkaline drug in alkaline drug in alkaline media→Less ionization of drug→Less water soluble →More lipid soluble→More absorption.


  • Surface area of absorption:More surface area→More absorption.For this reason,absorption is more in small intestine.
  • Vascularity: More vascularity→More absorption.Less vascularity→Less absorption.
  • Gastrointestine diseases:
  • Gut edema in CCF,CLD→↓Absorption.


  • Presence of food in gut:Food enhances the absorption of some drugs like riboflavin, propranolol.So these drugs are taken during meal.


  • Inactivation:Al,Na,Insulin,Oxytocin→Metabolized in gut,if given orally→Less/no absorption.

General principles of drug absorption:

Lipid solubility:

  • More lipid soluble→More absorption.
  • Less lipid soluble→Less absorption.


  • More ionization→more water soluble→Less absorption.
  • Less ionization→Less water soluble→More absorption.

Acidity or alkaline nature of the environment:

  • Acidic groups become less ionized in an acidic media.
  • Basic groups become less ionized in an basic media.

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