Route of Drug Administration

Factors affecting route of drug administration

1. Characteristics of the drug.

2. Emergency/routine use.

3. Site of action of the drug—local or systemic.

4. Condition of the patient (unconscious, vomiting, diarrhea).

5. Age of the patient.

6. Effect of gastric pH, digestive enzymes, and first-pass metabolism.

7. Owner’s/doctor’s choice (sometimes).

Local Routes

It is the simplest mode of administration of a drug at the site where the desired action is required.

Systemic side effects are minimal.

1. Topical:

The drug is applied to the skin or mucous membrane at various sites for local action.

Oral cavity:

As a suspension, e.g. nystatin; as a troche, e.g. clotrimazole (for oral candidiasis); as a cream, e.g. acyclovir (for herpes labialis); as ointment and jelly, e.g. 5% lignocaine hydrochloride (for topical anesthesia); as a spray, e.g. 10% lignocaine hydrochloride (for topical anesthesia).

GI tract:

As tablet that is not absorbed, e.g. neomycin (for sterilization of gut before surgery).

Rectum and anal canal:

i. As an enema (administration of a drug into the rectum in liquid form):

• Evacuant enema (for evacuation of the bowel): For example, soap water enema—soap acts as a lubricant, and water stimulates the rectum.

• Retention enema: For example, methylprednisolone in ulcerative colitis.

ii. As a suppository (administration of the drug in a solid form into the rectum), e.g. bisacodyl— for evacuation of bowels.

Eye, ear, and nose:

As drops, ointments, and sprays (for infection, allergic conditions, etc.), e.g. gentamicin eye/ear drops.

Bronchi:

As an inhalation, e.g. salbutamol, ipratropium bromide, etc. (for bronchial asthma and chronic obstructive pulmonary disease).

Skin:

As an ointment, cream, lotion, or powder, e.g. clotrimazole (antifungal) for cutaneous candidiasis.

2. Intra-arterial route:

This route is rarely employed. It is mainly used during diagnostic studies such as coronary angiography and for the administration of some anticancer drugs, e.g. for treatment of malignancy involving limbs.

3. Administration of the drug into some deep tissues by injection, e.g. administration of triamcinolone directly into the joint space in rheumatoid arthritis.

Systemic Routes

Drugs administered by this route enter the blood and produce systemic effects.

Enteral Routes

It includes oral, sublingual, and rectal routes.

Oral Route

It is the most common and acceptable route for drug administration. Dosage forms are tablet, capsule, syrup, mixture, etc., e.g., paracetamol tablet for fever, omeprazole capsule for peptic ulcer are given orally.

Advantages

• Safer.
• Cheaper.
• Painless.
• Convenient for repeated and prolonged use.

Disadvantages

• Not suitable for emergency as the onset of action of orally administered drugs is slow.
• It is not suitable for/in:
• Unpalatable and highly irritant drugs.
• Unabsorbable drugs (e.g. aminoglycosides).
• Drugs that are destroyed by digestive juices (e.g. insulin).
• Drugs with extensive first-pass metabolism (e.g. lignocaine).
• Unconscious patients.
• Uncooperative and unreliable patients.
• Patients with severe vomiting and diarrhea.

Sublingual Route

The preparation is kept under the tongue. The drug is absorbed through the buccal mucous membrane and enters the systemic circulation directly, e.g. nitroglycerin for acute anginal attack and buprenorphine for myocardial infarction.

Advantages

• The quick onset of action.
• Action can be terminated by spitting out the tablet.
• Bypasses first-pass metabolism.

Disadvantages

• It is not suitable for:
• Irritant and lipid-insoluble drugs.
• Drugs with bad smell and taste.

Rectal Route

Drugs can be given in the form of solid or liquid.

1. Suppository: It can be used for local (topical) effects (see p. 4) as well as systemic effects, e.g. indomethacin for rheumatoid arthritis.
2. Enema: Retention enema can be used for local effect (see p. 4) as well as systemic effect. The drug is absorbed through the rectal mucous membrane and produces a systemic effect, e.g. diazepam for status epilepticus in children.

Parenteral Routes

Routes of administration other than the enteral route are called parenteral routes.

Advantages

• The onset of action of drugs is faster; hence it is suitable for emergencies.
• Useful in:
  • Unconscious patient.
  • Uncooperative and unreliable patients.
  • Patients with vomiting and diarrhea.
• It is suitable for:
  • Irritant drugs.
  • Drugs with high first-pass metabolism.
  • Drugs not absorbed orally.
  • Digestive juices destroy drugs.

Disadvantages

• Require aseptic conditions.
• Preparations should be sterile and are expensive.
• Painful, invasive techniques.
• Can cause local tissue injury to nerves, vessels, etc.

Inhalation

Volatile liquids and gases are given by inhalation for systemic effects, e.g. general anaesthetics.

Advantages

• The quick onset of action.
• The dose required is very less, so systemic toxicity is minimized.
• The amount of drugs administered can be regulated.

Disadvantages

• Local irritation may cause increased respiratory secretions and bronchospasm.

Injections

Intradermal route:

The drug is injected into the layers of the skin, e.g. Bacillus Calmette–Guerin (BCG) vaccination and drug sensitivity tests. It is painful and only a small amount of the drug can be administered.

Subcutaneous (s.c.) route:

The drug is injected into the subcutaneous tissues of the thigh, abdomen, and arm, e.g. adrenaline, insulin, etc.

Advantages

• Self-administration is possible (e.g. insulin).
• Depot preparations can be inserted into the subcutaneous tissue

Disadvantages

• It is suitable only for nonirritant drugs.
• Drug absorption is slow; hence it is not suitable for emergencies.

Intramuscular (i.m.) route:

Drugs are injected into large muscles such as the deltoid, gluteus maximus, and vastus lateralis, e.g. paracetamol, diclofenac, etc. A volume of 5–10 mL can be given at a time.

Advantages

• Absorption is more rapid as compared to the oral route.
• Mild irritants, depot injections, soluble substances, and suspensions can be given by this route.

Disadvantages

• Aseptic conditions are needed.
• Intramuscular injections are painful and may cause an abscess.
• Self-administration is not possible.
• There may be an injury to the nerves.

Intravenous (i.v.) route:

Drugs are injected directly into the bloodstream through a vein. Drugs are administered as:

• Bolus: Single, relatively large dose of a drug injected rapidly or slowly as a single unit into a vein.

For example, i.v. ranitidine in bleeding peptic ulcer.

• Slow intravenous injection: For example, i.v. morphine in myocardial infarction.

• Intravenous infusion: For example, dopamine infusion in cardiogenic shock; mannitol infusion in cerebral edema; fluids infused intravenously in dehydration.

Advantages

• Bioavailability is 100%.
• The quick onset of action; therefore, is the route of choice in emergency, e.g. intravenous diazepam to control convulsions in status epilepticus.
• A large volume of fluid can be administered, e.g. intravenous fluids in patients with severe dehydration.
• Highly irritant drugs, e.g. anticancer drugs can be given because they get diluted in the blood.
• The hypertonic solution can be infused by the intravenous route, e.g. 20% mannitol in cerebral oedema.
• By i.v. infusion, a constant plasma level of the drug can be maintained, e.g. dopamine infusion in cardiogenic shock.

Disadvantages

• Once the drug is injected, its action cannot be halted.
• Local irritation may cause phlebitis.
• Strict aseptic conditions are needed.
• Extravasation of some drugs can cause injury, necrosis, and sloughing of tissues.
• The drug should usually be injected slowly.
• Before injecting, make sure that the tip of the needle is in the vein.

Intrathecal route:

The drug is injected into the subarachnoid space (spinal anesthetics, e.g. lignocaine; antibiotics, e.g. amphotericin B, etc.).

Intra-articular route:

The drug is injected directly into the joint space, e.g. hydrocortisone injection for rheumatoid arthritis. Strict aseptic precautions should be taken. Repeated administration may cause damage to the articular cartilage.

Transdermal route:

The drug administration is in the form of a patch or ointment that delivers the drug into the circulation for systemic effect.

Examples are a scopolamine patch for sialorrhoea and motion sickness, a nitroglycerin patch/ointment for angina, and an estrogen patch for hormone replacement therapy (HRT).

Advantages

• Self-administration is possible.
• Patient compliance is better.
• Prolong duration of action.
• Minimal side effects.
• Provides a constant plasma concentration of the drug.

Disadvantages

• Expensive.
• Local irritation may cause dermatitis and itching.
• The patch may fall off unnoticed