Routes of Drug Administration

FACTOR GOVERNING THE CHOICE OF ROUTE

1. Drug properties

• Physical form: Whether the drug is a solid, liquid, or gas determines if it can be given orally, inhaled, or injected.
• Solubility and stability: Drugs that are poorly water-soluble or unstable in gastric acid may need parenteral, sublingual, or transdermal routes.
• pH and irritancy: Highly irritant or very acidic/alkaline drugs may damage skin or mucosa and therefore must be given by a route that minimizes local injury (e.g., deep I.M. instead of S.C.).

2. Site and type of action

• Local action: When effect is required at a specific site (eye, skin, throat), local routes such as topical, ocular, or inhalational are preferred to limit systemic exposure.
• Systemic or generalized action: For actions needed throughout the body (antibiotics, antihypertensives), systemic routes (oral, I.V., etc.) are chosen, especially when the target site is not directly approachable.

3. Absorption characteristics

• Rate of absorption: Routes like intravenous give an immediate effect, while oral or transdermal routes produce slower, more sustained absorption.
• Extent of absorption: Some routes have variable or incomplete absorption (e.g., rectal, oral in the presence of food), which influences dose and reliability of effect.

4. Effect of GIT and first‑pass metabolism

• Digestive juices and enzymes: Acid, enzymes, and bile may degrade sensitive drugs (e.g., some peptides), making oral administration unsuitable.
• First‑pass hepatic metabolism: Drugs extensively metabolized in the liver after oral absorption may show low bioavailability, so alternative routes such as sublingual, transdermal, or parenteral are chosen.

5. Required speed of response

• Emergencies: Conditions like anaphylaxis, cardiac arrest, or acute asthma require a very rapid onset; intravenous or inhalational routes are preferred.
• Routine or chronic therapy: For long-term management (e.g., hypertension, diabetes), convenient routes such as oral or transdermal are favored even if onset is slower.

6. Accuracy and control of dosage

• High precision needed: Intravenous infusion allows very accurate control of dose and blood levels, important for drugs with a narrow therapeutic index.
• Less critical dosing: When small fluctuations are acceptable, oral or other convenient routes are used, even if absorption varies somewhat.

7. Patient’s condition and cooperation

• Consciousness and swallowing ability: Unconscious, vomiting, or uncooperative patients cannot take oral drugs, so parenteral, rectal, or transdermal routes are preferred.
• Age and special populations: Paediatrics, geriatrics, or patients with severe pain or phobia of injections may need modified routes (e.g., oral liquids, suppositories, patches) to improve comfort and adherence.

ORAL ROUTE

• Oral route means the drug is taken by mouth and swallowed, so it passes through the gastrointestinal (GI) tract before absorption.
• It is the most commonly used route in clinical practice because it suits long‑term therapy and outpatient use.

Key advantages

• Safe: Generally, the safest route because drug entry is slower and, if needed, the drug can sometimes be removed (induced vomiting, gastric lavage, activated charcoal).
• Convenient: Patients can self-administer tablets, capsules, or liquids at home without medical help, so treatment becomes pain-free, non-invasive, and easy to continue for chronic diseases.
• Economical: Oral formulations are cheaper to manufacture and administer than most injectable preparations, reducing overall treatment cost.
• Good absorption: The drug can be absorbed at multiple sites (stomach, small and large intestine), giving a large effective surface area and usually satisfactory bioavailability for many drugs.
• No sterilization required: Unlike injections, oral solid and liquid preparations do not need to be sterile, which simplifies production and storage and lowers cost.

Key Disadvantages

• Slow absorption and onset: Drug must disintegrate, dissolve, and pass through the GI tract, leading to 30 minutes to 2-hour delay—unsuitable for emergencies like cardiac arrest or severe pain.
• Nausea/vomiting and unpalatable drugs: Patients with nausea, vomiting, or motion sickness cannot retain tablets; bitter or large tablets reduce compliance.
• Unconscious or uncooperative patients: Comatose, post-operative, or paediatric patients unable to swallow safely require alternative routes like IV or rectal.
• Destruction by GI secretions: Acid-labile drugs (e.g., penicillin G, insulin) or enzyme-sensitive peptides are degraded in stomach acid or intestinal enzymes before absorption.
• Inefficient or variable absorption: Some drugs have poor bioavailability (e.g., due to pH changes, food effects, or disease states like diarrhea), requiring higher doses.
• First-pass metabolism: Orally absorbed drugs pass through the liver via the portal vein, undergoing extensive metabolism (e.g., propranolol, morphine), reducing systemic availability.
• Food-drug and drug-drug interactions: Food delays gastric emptying and absorption; drugs like tetracycline bind to dairy calcium, while antacids alter pH and reduce efficacy of others.