Routes of Drug Administration

Introduction: Route of administration is an important factor which influences the absorption of a drug. The interval between administration and onset of action is determined by the route of administration. Biological lag is the interval between administration of a drug and development of response.

Classification of routes: The routes of drug administration can be classifies as:

Oral or enteral route: It is most commonly used route for drug administration.

Advantages of oral route:

1. It is a safe, convenient and economical route.

2. Self medication is possible.

3. Withdrawal of the drug is possible.

Disadvantages of oral route:

1. Onset of drug action is slow.

2. Drugs which are bitter in taste cannot be administered.

3. Drugs producing nausea and vomiting cannot be administered.

4. The drug may be inactivated by gastric enzymes.

5. This route is not possible in an unconscious patient.

Enteric coated pills and tablets: These are oral preparations coated with cellulose acetate or gluten. These coatings cannot be destroyed by the acid juice of the stomach. Only the alkaline intestinal juice removes these coatings. So inactivation of the drug in the stomach is avoided. Thus a desired concentration of the drug is released in the intestine.

Sustained release or time release preparations (Spansules): These are oral preparations containing various coatings. Each coating dissolves at different time intervals releasing the active drug. So the drug is released slowed for prolonged periods.

Parenteral routes: Routes of administration other than oral (enteral) route are termed as parenteral.

Advantages of parenteral route:

1. Absorption is rapid and quick.

2. Accurate dose of the drug can be given.

3. The drug enters into circulation in an active form.

4. It is useful in emergency.

5. It is useful in case of an unconscious patient.

Disadvantages of parenteral route:

1. Pain may be produced by injection.

2. Abscess procedures are required for injection.

3. Sterile procedures are required for injection.

4. It is an expensive route.

5. Self medication is not possible.

a. Injection

1. Intradermal: The drug is injected in the layers of skin e.g. B.C.G vaccine.

2. Subcutaneous: Non-irritant substance alone can be injected by this route. The rate of absorption is even and slow and hence the effect is prolonged.

3. Intramuscular: The drug is injected deep into muscle tissue. The rate of absorption is uniform and onset of action is rapid.

4. Intravenous: A drug is directly injected into a vein.

Advantages:

(i) The drug enters into circulation in an active form.

(ii) Desired blood concentration can be obtained.

(iii) Quick and immediate effect is produced.

(iv) It is useful in case of emergency.

(v) It is useful in an unconscious patient.

Disadvantages:

(i) Drugs which precipitate blood constituents cannot be administered.

(ii) Untoward reactions, if occur are immediate.

(iii) Withdrawal of the drug is not possible.

5. Intra-arterial: In this route, a drug is injected into an artery. The effect of a drug can be localized in a particular organ or tissue by choosing the appropriate artery. Anticancer drugs are sometimes administered by this route.

6. Intraperitoneal: In this route, a drug is injected into the peritoneal cavity. By this, fluid like glucose and saline can be given to children.

7. Bone - marrow: Bone marrow injection is very similar to intravenous injection. This route is useful when veins are not available due to circulatory collapse or thrombosis. In adults, the sternum is chosen and in children, tibia or femur is chosen for injection.

b. Inhalation: Gases, volatile liquids, aerosols or vapours can be administered by this route.

Advantages:

1. Immediate absorption of the drug.

2. Localization of the effect in diseases of the respiratory tract.

Disadvantages:

1. Poor ability to regulate the dose.

2. Local irritation of the respiratory tract may increase its secretions.

3. Difficulty in the method of administration.

c. Transacutaneous route: It is further classified as

i) Iontophoresis

ii) Inunctions

iii) Jet injection

iv) Adhesive units.

i) Iontophoresis: In this method, a drug is driven deep into the skin by means of a galvanic current e.g. salicylates. Anode iontophoresis is used for positively charged drugs and cathode iontophoresis is use for negatively charged compounds.

ii) Inunction: It is rubbing the drug on the skin. The drug gets absorbed and produces systemic effect e.g. nitroglycerine ointment for angina.

iii) Jet injection: This method does not require a spring. So it is painless. Using a gun like instrument with a micro-fine orifice, the drug solution is projected as a high velocity jet (dermojet). The drug solution passes through superficial layers of skin and gets deposited in the subcutaneous tissue. This method is useful for mass inoculation.

iv) Adhesive units: It is a trans-dermal drug delivery system. It is available in the form of adhesive unit. It delivers the drug slowly. So it produces prolonged systemic effect. e.g. scopolamine for motion sickness.

d. Transmucosal route: It is further classified as

A) Sub-lingual

B) Trans-nasal

C) Trans-rectal.

A) Sub-lingual route: A tablet containing the drug is put under the tongue and allowed to dissolve in the mouth e.g. nitroglycerine and isoprenaline.

Advantages of sub-lingual route:

1. Rapid onset of action.

2. Termination of the effect by spitting the tablet.

3. Inactivation of the drug in the stomach is avoided.

4. The drug enters directly into systemic circulation without inactivation in the liver.

B) Trans-nasal route: It is useful for drugs in the form of snuff or nasal spray. The drug is readily absorbed through the mucous membrane of nose. e.g. posterior pituitary powder.

C) Trans-rectal route: Drugs can be absorbed through the rectum for producing systemic effects. e.g. aminophylline for broncho-spasm, diazepam for status epileptic-us. Advantages of rectal route are:

1. Gastric irritation is avoided.

2. It is useful in old and terminally ill patients.

e. New drug delivery systems:

i) Occusert: It is placed directly under the eyelid. It can release drugs like pilocarpine for prolonged periods.

ii) Progestasert: It is an intrauterine contraceptive device. It produces controlled release of progesterone within the uterus for a year.

iii) Pro-drug: It is an inactive drug which after administration is metabolized into an active drug. For example, I-dopa is an inactive compound. After administration, it is metabolized to the active drug dopamine which is effective in Parkinson.

Local application: Drugs in the form of powder, paste, lotion, drops and ointment can be applied locally for action at the site of application. Drugs can be applied on mucous membranes of nose, conjunctiva, vagina, urethra and rectum. The following are some preparations which are meant for local application:

1. Bougie for urethra.

2. Pessary for vagina.

3. Suppository for vagina and rectum.

4. Enemata for rectum.