Routes of Drug Administration

Why This Matters

When you're tested on pharmacology, you're not just being asked to list administration routes. You're being evaluated on whether you understand why a clinician would choose one route over another. The key principles at play here are bioavailability (how much drug actually reaches systemic circulation), onset of action (how quickly effects begin), and first-pass metabolism (the liver's processing of drugs before they reach the bloodstream). These concepts drive everything from emergency treatment protocols to chronic disease management.

Think of administration routes as a decision tree: Does the patient need immediate effects or sustained release? Can they swallow? Do we want local or systemic effects? Does the drug survive the GI tract? Don't just memorize that nitroglycerin goes under the tongue. Know that it's given sublingually because it would be largely destroyed by first-pass metabolism if swallowed. That's the level of understanding that earns full credit on exams.

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Routes That Bypass First-Pass Metabolism

First-pass metabolism occurs when drugs absorbed from the GI tract travel through the hepatic portal vein to the liver before reaching systemic circulation, often significantly reducing bioavailability. The routes below deliver drugs into the bloodstream while avoiding that hepatic processing.

Sublingual Administration

• Placed under the tongue for absorption through the highly vascular sublingual mucosa, with onset typically within 1–3 minutes
• Bypasses first-pass metabolism, making it ideal for drugs like nitroglycerin that would be extensively broken down by the liver if swallowed
• Requires patient cooperation: the drug must dissolve completely under the tongue without swallowing or chewing for maximum effect

Buccal Administration

• Placed between the gum and cheek, absorbing through the buccal mucosa into systemic circulation
• Slower onset than sublingual because the buccal mucosa is less permeable, but this makes it useful for sustained-release formulations
• Avoids GI degradation: commonly used for fentanyl buccal tablets and certain hormone therapies

Transdermal Administration

• Patches deliver medication through the skin into dermal capillaries for continuous, controlled systemic absorption
• Bypasses first-pass metabolism and maintains steady plasma levels over 24–72 hours, reducing peak-and-trough fluctuations
• Limited to lipophilic (fat-soluble) drugs with low molecular weight: examples include fentanyl, nicotine, and estradiol patches

Intranasal Administration

• Absorbed through the nasal mucosa directly into systemic circulation, with onset typically in 10–15 minutes
• Bypasses first-pass metabolism: critical for emergency medications like naloxone (Narcan) for opioid overdose reversal
• Rich blood supply allows rapid absorption; also used for sumatriptan (migraine treatment) and certain flu vaccines

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Parenteral Routes: Direct Systemic Access

Parenteral administration refers to any route that bypasses the gastrointestinal tract entirely. These injectable routes offer predictable absorption and are essential when oral administration is impossible or inadequate.

Intravenous (IV) Administration

• 100% bioavailability: drug enters the bloodstream directly with immediate onset of action
• Allows precise titration of drug levels and is essential for emergency situations, continuous infusions, and drugs that are too irritating for IM or SC injection
• Highest risk for adverse reactions: once administered, the drug cannot be retrieved. Requires sterile technique and careful rate monitoring

Intramuscular (IM) Injection

• Injected into muscle tissue (deltoid, vastus lateralis, ventrogluteal) where rich blood supply enables absorption in roughly 10–30 minutes
• Accommodates larger volumes (up to 3–5 mL depending on site) and depot formulations for prolonged release (e.g., long-acting antipsychotics like haloperidol decanoate)
• Absorption rate varies with blood flow: exercise and warmth increase absorption, while shock or vasoconstriction decrease it

Subcutaneous (SC/SubQ) Injection

• Injected into the adipose tissue beneath the dermis, providing slower, more sustained absorption than IM
• Ideal for self-administration: the standard route for insulin, low-molecular-weight heparin (enoxaparin), and some vaccines
• Limited volume capacity (typically 1–2 mL); absorption is affected by blood flow, injection site rotation, and tissue perfusion

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Enteral Routes: Through the GI Tract

Enteral administration involves the gastrointestinal system. While convenient and cost-effective, these routes subject drugs to digestive enzymes, variable pH environments, and first-pass metabolism, all of which affect how much active drug ultimately reaches the bloodstream.

Oral Administration

• Most common and convenient route: the patient swallows a tablet, capsule, or liquid for absorption primarily in the small intestine
• Subject to first-pass metabolism, which can significantly reduce bioavailability before the drug reaches systemic circulation. For example, oral nitroglycerin has very low bioavailability compared to the sublingual form
• Absorption affected by food, gastric pH, and GI motility: some drugs require an empty stomach for best absorption, while others need food to reduce GI irritation or improve absorption

Rectal Administration

• Suppositories or enemas inserted into the rectum for local effects (e.g., treating hemorrhoids) or systemic absorption through the rectal mucosa
• Partially bypasses first-pass metabolism: the lower rectal veins (inferior and middle) drain into the inferior vena cava rather than the hepatic portal system, so drugs absorbed low in the rectum avoid the liver
• Useful when the oral route is unavailable: vomiting patients, unconscious patients, or those with swallowing difficulties. However, absorption can be unpredictable

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Topical and Local Routes

These routes deliver medication directly to the site of action, minimizing systemic exposure and side effects. The goal is a therapeutic effect at the application site rather than throughout the body.

Topical Application

• Applied directly to the skin surface as creams, ointments, lotions, or gels for localized effect
• Minimal systemic absorption when skin is intact: treats conditions like dermatitis, fungal infections, and localized inflammation
• Absorption increases with broken or inflamed skin, occlusive dressings, or application to thin-skinned areas (e.g., eyelids, groin)

Ophthalmic Administration

• Drops, ointments, or gels applied to the conjunctival sac for local treatment of conditions like glaucoma, infections, and inflammation
• Systemic absorption can occur through nasolacrimal drainage into the nasal mucosa. Teach patients to apply pressure to the inner canthus (nasolacrimal duct) for 1–2 minutes after instillation to minimize this
• Proper technique is essential: avoid touching the dropper tip to the eye; wait at least 5 minutes between different eye medications to prevent washout

Otic Administration

• Medication instilled into the external ear canal for local treatment of infections, inflammation, or cerumen (earwax) impaction
• Proper positioning required: for adults, pull the pinna up and back; for children under age 3, pull down and back. This straightens the ear canal for proper delivery
• Temperature matters: cold solutions can cause vertigo by stimulating the vestibular system, so warm drops to near body temperature before instilling

Vaginal Administration

• Inserted as creams, tablets, suppositories, or rings for local or sometimes systemic effects
• Common uses include antifungal treatments (miconazole, clotrimazole), hormonal therapy (estrogen rings), and labor induction (prostaglandins like dinoprostone)
• Absorption varies with vaginal pH, stage of menstrual cycle, and presence of infection or inflammation

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Inhalation Route: Pulmonary Delivery

The lungs offer an enormous surface area (approximately 70 square meters) with extremely thin alveolar membranes and rich blood supply. This enables rapid absorption for both local respiratory effects and, in some cases, systemic delivery.

Inhalation

• Medication delivered as metered-dose inhalers (MDIs), dry powder inhalers (DPIs), or nebulizers directly to the respiratory tract
• Primary use is respiratory conditions: bronchodilators (albuterol) for asthma, inhaled corticosteroids (fluticasone) for COPD and asthma maintenance, mucolytics for thick secretions
• Lower doses achieve therapeutic effect compared to systemic administration, which significantly reduces systemic side effects. For example, inhaled corticosteroids cause far fewer side effects than oral corticosteroids at equivalent therapeutic doses

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Quick Reference Table

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Self-Check Questions

1. A patient experiencing an acute angina attack needs immediate relief. Why is nitroglycerin given sublingually rather than orally, and what pharmacokinetic principle does this illustrate?

2. Compare and contrast IM and SC injection routes: What factors would lead a clinician to choose one over the other for a specific medication?

3. Which three administration routes would be appropriate for an unconscious patient who cannot swallow, and what are the advantages and limitations of each?

4. A question describes two products: a fentanyl patch for chronic pain and a hydrocortisone cream for a localized rash. Explain why these represent fundamentally different administration concepts despite both being applied to the skin.

5. Explain why the same drug might require different doses depending on the administration route. Which routes would demonstrate the greatest dose differences, and why? (Hint: think about bioavailability and first-pass metabolism.)