Drug absorption simply means transport of drug from site of application to site of systemic circulation.

As the drug reaches the systemic circulation the drug flows with the blood and also reaches many cells, tissues, organs and along with the site of action.

Drug absorption and drug distribution and likewise factors affecting and many terminologies related to this we will study below.

What you get below?

  1. Drug Absorption
  2. Factors affecting absorption
  3. Absorption from various routes
  4. Bioavailability
  5. Drug Distribution
  6. Terminologies in drug distribution
  7. Protein binding
  8. Conclusion
  9. Frequently Asked Questions (FAQs)


Drug absorption is defined as movement of drug from site of administration to the systemic circulation.

If you see rate of absorption determines the Absorption of drug. Rate of absorption depends on concentration gradient, molecular weight, biological membrane, first pass metabolism, size, shape, and also genetic factor.

The drugs given by intravenous route of drug administration the bioavailability is 100% (as discussed in earlier blog and also we will learn further), this is due to the drug directly reaches systemic circulation it don’t have to cross and biological membrane.

Factors affecting absorption

  1. Concentration: As discussed in earlier blog, passive transport depends on concentration gradient that means the drug travel from high concentration region to low concentration region without utilizing energy. So, if the drug given in concentrated for will absorb readily than given in dilute form.
  2. Aqueous solubility: Drugs given in solid dosage forms like tablets, capsules, etc should dissolve in aqueous bio-liquid first and then get absorbed into systemic fluid. Above all rate of absorption is directly dependent on rate of dissolution. liquid dosage form absorbs drug readily.
  3. Area of absorbing surface: larger the area faster the absorption.
  4. Flow of blood to the site of administration: More the blood flow to the site of administration, the more is the absorption because the blood circulation maintains the concentration gradient. If the blood circulation is very fast then it may hasten the drug concentration because drug usually flows with the blood flow.
  5. Route of Drug administration: The Intravenous route has highest preference because it releases the drug directly into the systemic circulation. Oral route and topic route has the lowest preference because it has to pass first pass metabolism and biological membranes. The advantages and disadvantages vary according to the variation in the route of administration.

Absorption from various routes

1) Oral :

When the drug is taken from oral route, the most important barrier for the drug is Epithelial lining of GIT (gastro intestinal tract), which to clarify is lipoidal in nature.

As the drug reaches stomach there is gastric juice which is acidic in nature and as a result only acidic drugs gets solublizes into it and absorbed readily. A basic drug gets solublizes in duodenum due to basic nature of duodenum.

For Example Acidic drugs are Salicylates and barbiturates while that of basic drugs are Morphine and quinine.

There may be hurdle for acidic drugs too in stomach due to mucus lining; so the absorption is slower when mucus lining is present.

The function of mucus lining is that it protects our internal lining of stomach from acidic gastric juice.

Presence of food also affects drug absorption because drug gets soluble in the food that we have taken.

Enteric coated drugs are used to overcome the effects of certain factors, to clarify this coat is acid resistant.

2) Subcutaneous and Intramuscular :

Drug is administered or is inserted in very close vicinity to the capillaries, as a result of this, the lipid soluble drugs gets absorbed into the systemic circulation from close capillary.

The Intramuscular absorption is faster as compared to subcutaneous absorption. But as compared to oral absorption these both route are more preferred.

Subcutaneous and also intramuscular absorption is enhanced by applying heat to the site of administration while vasoconstrictor like adrenaline retards the absorption.

3) Topical sites:

Topical sites for instance include skin, cornea, mucus membranes, etc.

It is very slow method of absorption this is because the site is intact and lipoidal; the absorption can be consequently enhanced by increasing heat or by rubbing on the site of application of drug.

Cornea is permeable to lipid soluble drugs and similarly is mucus membrane. Abraded surface rapidly absorbs drugs.


When the drug is administered from various routes of drug administration (except intravenous) lot amount of drug is wasted till it reaches site of action. The fraction of amount of drug reaches the systemic circulation in unchanged is known is bioavailability.

The intravenous route has 100% bioavailability this is as a result of the drug is directly injected into the systemic circulation.

Clinical significance:

  1. It is used to decide route of drug administration.
  2. It is used to determine dose of drug to be given.
  3. Used to determine which dosage form of the same drug to be given.
  4. Onset of action.
  5. Duration of action.
  6. Bioequivalence of drug.


As the drug gets into systemic circulation, due to flow of blood the drug reaches many cells, tissues, organs and site of action.

Drug reaches to many sites along the concentration gradient. As the plasma has more amount of drug, that is it has high concentration of drug, it reaches to cells and tissues having no drug concentration.

The drug distribution depends on:

  1. lipid solubility
  2. Ionization
  3. Binding capacity of drug with proteins.
  4. Affinity of drug for different tissues.
  5. Body mass ratio.
  6. Diseased conditions.

The movement of drug depends upon the equilibrium between the unbound drug in plasma and Also in tissue fluid.

As the elimination proceeds, there is equal decrease in concentration between both.

Terminologies in drug distribution

1) Apparent volume of distribution (V):

It is the volume of drug present in whole body as compared to volume present in plasma.

Volume of distribution (Vd) also referred to as apparent volume of distribution is explained as after, absorption of a drug a fraction gets attached to plasma protein and fraction is unbound or remains free.

The unbound or free drug distributes to many sites like cell and tissues the fraction of drug which gets distributed is known as volume of distribution. This term do not indicate real volume, it just indicates the fraction of drug which is in free form.

Example: suppose 300mg of drug is administered i.v and the plasma concentration produced is suppose 10 mg/L. the apparent volume of distribution will be 300/10 = 30L.

2) Redistribution:

When the drug reaches systemic circulation, the drug distributes to those sites or organs where the blood circulation is fastest, and then to other sites. As the concentration of drug reduces in the blood, the drug from the highest concentration comes back into the blood circulation this whole phenomenon is known as redistribution.

Plasma Protein Binding (PPB)

After absorption of drug into systemic circulation, the fraction of drugs gets free and fraction bound to plasma protein. The binding of drug to the plasma protein is known as Plasma Protein Binding.

Acidic drugs generally bind to plasma albumin and basic drugs to alpha glycoprotein. Extent of this bind varies according to drug concentration.

Example: 90% of absorbed fraction of doxycycline is bound to plasma protein and only 10% of gentamicin is bound to plasma protein.

Warfarin, Frusemide, Diphenylbutazone, Diazepam are drugs which have high affinity to plasma protein while Amoxycillin and ethosuccimide are drugs having low affinity.

  1. The protein bound drug is pharmacologically inactive and only free drug is active.
  2. Volume of distribution is inversely proportional to protein binding this is because the protein bound drug will not distribute throughout the body on free drug can be distributed.
  3. Protein bound drug serves as the reservoir and can be used for prolonged action.
  4. Protein bound drug and free drug are in equilibrium.
  5. In diseased state like hepatic failure, the plasma protein binding is totally reduced.
  6. If two drugs which have same plasma protein binding site, then the drug having more affinity will succeed.
  7. Protein bound drugs become so large in size that they cannot cross the biological membrane.

Acidic and basic drugs which bind to proteins are:

or AlbuminFor α-glycoprotein
Barbituric acidLignocaine or lidocaine
Valproic acidMethadone


Here we have learnt, a part of pharmacology; how the drug gets absorbed into systemic circulation and how drugs distributes thorough the body and produce the desired response.

Bioavailability measures rate and extent of drug absorption. Disease can alter the bioavailability.

Enterohepatic circulation may prolong the effect of drug.

Therapeutic concentration is one that determines the concentration, onset and duration of action of drug.

Route of administration, onset of action, duration of action, Bioequivalence, Dosage form determines the bioavailability. Volume of distribution determines the free drug.

Protein binding reduces the drug distribution. Drug action prolongs if the drug gets redistributed.

Frequently Asked Questions (FAQs)

1. Which drugs gets absorbed in intestine?

Generally a basic drug gets absorbed from intestine.

2. Where do drug go after absorption?

Drug goes in to systemic circulation after absorption.

3. Which drugs gets absorbed in stomach?

Generally a acidic drug gets absorbed from stomach.

4. Drug absorption in adult’s vs. Children?

Drug absorption is more in adults because all the organ system are more developed as compared to children.

5. How blood flow affects drug distribution?

As the drug reaches systemic circulation, the force of blood takes the drug molecule with it and drug travels where the blood takes it.

6. How obesity affects drug distribution?

If the person is obese the person need more amount of drug concentration.

7. Drug distribution vs. protein binding?

When the drug is in Free State the drug gets distributed, but when drug is in bound state the drugs complex acts as drug reservoir.

8. Where the drug reaches first?

Drug reaches first to liver most probably because the first pass metabolism is carried out their.



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