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Routes of Drug Administration

The path taken by the drug to get into the body is known as the route of drug administration. A drug may be in ionized or unionized form.

Contents
Enteral Route
Parenteral Route
Inhalation
Topical Route

Classification:

  1. Enteral route
  2. Parenteral route
  3. Inhalation
  4. Topical

1. Enteral Route:

Enteral route is through the alimentary canal. It might be:

  1. Oral
  2. Sublingual
  3. Per rectum

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a. Oral Route:

Oral route is the most common route of drug administration. It is mostly used for the neutral drugs. It may be in the form of tablets, capsules, syrup, emulsions or powders.

Advantages:
  1. It is convenient
  2. It is the cheapest available route
  3. It is easy to use
  4. It is safe and acceptable.
Disadvantages:
  1. Less amount of drug reaches the target tissue.
  2. Some of the drug is destroyed by gastric juices e.g. adrenaline, insulin, oxytocin
  3. Absorption has to take place which is slow, so is not preferred during emergency.
  4. It might cause gastric irritation
  5. It might be objectionable in taste.
  6. It might cause discoloration of teeth e.g. iron causes staining, tetracyclines below 14 cause brown discoloration so are not advisable during pregnancy.
First Pass Effect:

First pass effect is the term used for hepatic metabolism of drug when absorbed and delivered through portal blood. Greater the first pass effect, less amounts of the drug reach the systemic circulation.

b. Sublingual Route:

Sublingual route involves tablets placed under the tongue or between cheeks or Gingiva. The drug should be lipid soluble and small.

Advantages:
  1. Rapid absorption takes place.
  2. Drug is dissolved easily
  3. Drug enters the blood directly
  4. Less first pass effect.
  5. Spitting out of the drug removes its effect
Disadvantages:
  1. This method is inconvenient.
  2. Irritation of the mucous membrane might occur
  3. Person may swallow the drug
  4. Might be unpleasant in taste.

Examples of drugs given by this route include nitroglycerin, isoprenaline and oxytocin. Nifedipine used for the treatment of hypertension in emergency is given by sublingual route.

c. Rectal Route:

Drugs in solid forms such as suppositories or in liquid forms such as enema are given by this route. This route is mostly used in old patients. Drugs may have local or systemic actions after absorption.

Advantages:
  1. This route is preferred in unconscious or uncooperative patients.
  2. This route avoids nausea or vomiting
  3. Drug cannot be destroyed by enzymes.
  4. This route is preferred if drug is irritant.
Disadvantages:

This route is generally not acceptable by the patients.

Locally acting drugs include glycerin and Bisacodyl suppository

Systemic acting drugs include Indomethacin (anti inflammatory) and aminophyllin (bronchodilator)

Retention enema is diagnostic and is used for finding the pathology of lower intestines.

Drugs given by rectal route have 50% first pass metabolism.


2. Parenteral Route:

Parenteral route includes:

Injections:

  1. Intra muscular
  2. Intra venous
  3. Intra-arterial
  4. Intra-cardiac
  5. Intra-thecal
  6. Intraosseous- into bone marrow
  7. Intrapleural
  8. Intraperitoneal
  9. Intra-articular
  10. Intradermal (Intracutaneous)
  11. Subcutaneous route (Hypodermic)

Inhalation

Hypospray or jet injections

Advantages:
  1. Parenteral route is rapid.
  2. It is useful for uncooperative patients
  3. It is useful for unconscious patients
  4. Inactivation by GIT enzymes is avoided
  5. First pass effect is avoided
  6. Bioavailability is 100%
Disadvantages:
  1. Skill is required
  2. It is painful
  3. This method is expensive
  4. It is less safe.
Classification:
Site of Release:

Site of release may be intradermal, intraperitoneal, intrapleural, intracardiac, intra-arterial, intrathecal (into meninges of spinal cord), intra-articular (into joint cavity).

a. Subcutaneous:

Subcutaneous route might be used for the arm, forearm, thigh and subscapular space.

The volume used is 2 ml. Insoluble suspensions like insulin and solids might be applied by this route.

Advantages:
  1. Absorption is slow and constant
  2. It is hygienic
Disadvantages:
  1. It might lead to abscess formation
  2. Absorption is limited by blood flow

Examples of drugs given by subcutaneous route include insulin, adrenaline and norplant.

b. Intramuscular route:

Intramuscular route might be applied to the buttock, thigh and deltoid. The volume used is 3 ml.

Advantages:
  1. Absorption is rapid than subcutaneous route.
  2. Oily preparations can be used.
  3. Irritative substances might be given
  4. Slow releasing drugs can be given by this route.
Disadvantages

Using this route might cause nerve or vein damage.

c. Intravenous injections:

Intravenous injections might be applied to the cubital, basilic and cephalic veins.

Advantages:
  1. Immediate action takes place
  2. This route is preferred in emergency situations
  3. This route is preferred for unconscious patients.
  4. Titration of dose is possible.
  5. Large volume of fluids might be injected by this route
  6. Diluted irritant might be injected
  7. Absorption is not required
  8. No first pass effect takes place.
  9. Blood plasma or fluids might be injected.
Disadvantages:
  1. There is no retreat
  2. This method is more risky
  3. Sepsis-Infection might occur
  4. Phlebitis(Inflammation of the blood vessel) might occur
  5. Infiltration of surrounding tissues might result.
  6. This method is not suitable for oily preparations
  7. This method is not suitable for insoluble preparations
d. Intraarterial route:

This method is used for chemotherapy in cases of malignant tumors and in angiography.

e. Intradermal route:

This route is mostly used for diagnostic purposes and is involved in:

Schick test for Diphtheria

Dick test for Scarlet fever

Vaccines include DBT, BCG and polio

Sensitivity is to penicillin

f. Intracardiac  route

Injection can be applied to the left ventricle in case of cardiac arrest.

g. Intrathecal route:

Intrathecal route involves the subarachnoid space. Injection may be applied for the lumbar puncture, for spinal anesthesia and for diagnostic purposes. This technique requires special precautions.

h. Intra-articular route:

Intra-articular route involves injection into the joint cavity. Corticosteroids may be injected by this route in acute arthritis.

i. Intraperitoneal route:

Intraperitoneal route may be used for peritoneal dialysis.

j. Intrapleural route:

Penicillin may be injected in cases of lung empyma by intrapleural route.

k. Injection into bone marrow

This route may be used for diagnostic or therapeutic purposes.

Hypospray/Jet Injection:

This method is needleless and is subcutaneous done by applying pressure over the skin. The drug solution is retained under pressure in a container called ‘gun’. It is held with nozzle against the skin. Pressure on the nozzle allows a fine jet of solution to emerge with great force. The solution can penetrate the skin and subcutaneous tissue to a variable depth as determined by the pressure. Mass inoculation is possible but the method is expensive, definite skills are required and cuts might result.

3. Inhalation:

Inhalation may be the route of choice to avoid the systemic effects. In this way drugs can pass directly to the lungs. Drugs used involve volatile drugs and gases. Examples include aerosols like salbutamol; steam inhalations include tincture and Benzoin

Advantages:
  1. Rapid absorption takes place.
  2. Rapid onset of action takes place.
  3. This route has minimum side effects.
  4. No first pass effect takes place.
  5. This method is easy.
  6. Fewer doses is required.
Disadvantages:
  1. Special apparatus is required.
  2. Irritation of the respiratory tract may take place.
  3. Cooperation of the patient is required.
  4. Airway must be patent.

4. Topical route:

Drugs may be applied to the external surfaces, the skin and the mucous membranes. Topical route includes:

a. Enepidermic route

When the drug is applied to the outer skin, it is called enepidermic route of drug administration. Examples include poultices, plasters, creams and ointments.

b. Epidermic route (Innunition):

When the drug is rubbed into the skin, it is known as epidermic route. Examples include different oils.

c. Insufflations:

When drug in finely powdered form is blown into the body cavities or spaces with special nebulizer, the method is known as insufflations.

d. Instillation

Liquids may be poured into the body by a dropper into the conjunctival sac, ear, nose and wounds. Solids may also be administered.

e. Irrigation or Douching

This method is used for washing a cavity e.g. urinary bladder, uterus, vagina and urethra. It is also used for application of antiseptic drugs.

f. Painting/Swabbing

Drugs are simply applied in the form of lotion on cutaneous or mucosal surfaces of buccal, nasal cavity and other internal organs.

Time of Action using Different Routes of Administration

Drugs take different time durations after injection using different routes to perform their actions.

This time delay is important, oral route has controlled release time, thus depot or reservoir preparation may be made e.g. penicillin for rheumatic fever.

Usage of drug depends on its physical properties, chemical properties, speed of action, need and bypass effect.

Some of the approximate time intervals are given below:

Route of Drug Administration Delay time for Action
Intravenous route 30-60 seconds
Intraosseous route 30-60 seconds
Endotracheal inhalation 2-3 minutes
Sublingual route 3-5 minutes
Intramuscular route 10-20 minutes
Rectal route 5-30 minutes
Ingestion 30-90 minutes

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