Routes of Drug Administration — Chapter 4

Introduction

Chapter — Pharmacokinetics

Routes of Drug Administration

How a drug enters the body is just as important as what the drug does once it gets there. Choosing the right route determines onset, bioavailability, patient convenience — and sometimes whether the drug works at all.

Estimated read: 15 min 3 knowledge checks Core pharmacokinetics PharmD Year 1

Why route of administration matters

Before a drug can exert its pharmacological effect, it must reach its site of action at an adequate concentration. The route of administration is the gateway to that process.

The route you choose directly shapes the drug's pharmacokinetic profile — specifically its absorption rate, extent of bioavailability, and the time-to-peak plasma concentration (T_max). A drug that works beautifully intravenously might be completely ineffective orally if it is destroyed by gastric acid or so extensively metabolised on first pass through the liver that almost none reaches the systemic circulation.

Route selection is therefore a clinical decision, not a default. It must account for the drug's physicochemical properties, the urgency of treatment, the patient's clinical state, tolerability, and practical considerations including ease of self-administration and adherence.

Clinical pearl

Bioavailability (F) is defined as the fraction of the administered dose that reaches systemic circulation unchanged. IV administration always gives F = 1 (100%), making it the reference standard against which other routes are compared.

Major route categories

Enteral, Parenteral, Topical

Fastest onset route

Seconds to minutes

Highest bioavailability

100%

IV bolus, F = 1

Most common outpatient

Oral administration

Enteral routes

Enteral routes involve drug absorption through the gastrointestinal tract — the most natural and widely used pathway for drug delivery.

The GI tract provides an enormous absorptive surface area (estimated 200–300 m² when microvilli are accounted for) and a rich blood supply, making it highly effective for most small-molecule drugs. The trade-off is the complexity of the GI environment — variable pH, motility, enzymes, transporters, and first-pass hepatic metabolism all influence what ultimately enters the bloodstream.

Oral (PO — per os)

Oral administration is the most common, convenient, and cost-effective route. Tablets, capsules, liquids, and chewable formulations all deliver drug to the GI tract for absorption, predominantly in the small intestine where the large surface area and slightly alkaline pH favour absorption of weak acids and bases alike. The portal circulation then carries absorbed drug directly to the liver before it reaches systemic circulation — a critical point discussed under first-pass metabolism.

Clinical consideration

Gastric emptying rate is the single biggest variable affecting oral drug absorption. Food, opioids, and anticholinergic drugs all slow gastric emptying, delaying absorption. Metoclopramide, by contrast, accelerates it — relevant when rapid onset is needed.

Sublingual (SL) and buccal

Placing a drug under the tongue (sublingual) or against the buccal mucosa takes advantage of the rich capillary network directly beneath the oral mucosa. Absorption here bypasses the portal circulation entirely, delivering drug straight into the systemic venous drainage. Onset is rapid — often within 2–5 minutes — and the first-pass effect is avoided completely.

The classic example is glyceryl trinitrate (GTN), which is almost entirely destroyed by first-pass metabolism when swallowed but is highly effective sublingually for acute angina relief. Buprenorphine and fentanyl lozenges exploit similar principles in pain management.

Rectal (PR — per rectum)

Rectal administration is underutilised in many clinical settings, yet it offers distinct advantages. It is particularly valuable when oral administration is not possible — vomiting, unconsciousness, post-surgical patients, paediatric patients who refuse oral medications. The rectal mucosa is well-vascularised, and drugs absorbed from the lower rectum largely bypass the portal circulation, reducing first-pass metabolism compared with oral dosing. Higher rectal absorption, however, is more variable than oral due to inconsistent positioning, stool presence, and mucosal condition.

Example in practice

Diazepam rectal gel (Diastat) is a cornerstone of emergency seizure management in paediatric patients and community settings, precisely because it can be administered quickly without venous access.

Parenteral routes

Parenteral means literally "outside the gut." These routes bypass the GI tract entirely, offering precise control over absorption and dosing.

Parenteral routes are invaluable when rapid onset is required, when drugs are unstable in the GI environment, when a patient cannot take anything by mouth, or when 100% bioavailability is clinically necessary. The trade-off is invasiveness, risk of infection, need for sterility, and (usually) the requirement for a trained healthcare professional to administer the drug.

Intravenous (IV)

The intravenous route delivers drug directly into the bloodstream, guaranteeing 100% bioavailability and the fastest possible onset — often seconds for bolus doses. It allows precise dose titration and immediate response to adverse effects (to a point — you cannot recall an IV bolus once given). IV administration is essential in emergencies, intensive care, for drugs with poor oral bioavailability (e.g. aminoglycosides, vancomycin), and for continuous infusions requiring tight plasma concentration control.

IV formulations must be sterile, pyrogen-free, and generally isotonic. Drugs must be in solution or stable emulsion. Precipitation in the line, phlebitis, extravasation, and air embolism are recognised complications.

Intramuscular (IM)

Intramuscular injection deposits drug into skeletal muscle, from which absorption into capillaries occurs by diffusion. Onset is slower than IV (typically 10–30 minutes for aqueous solutions) but faster than oral. IM is an excellent route for depot formulations — oily or microcrystalline suspensions designed to release drug slowly over days to months. Long-acting injectable antipsychotics (LAIs) such as paliperidone palmitate and aripiprazole monohydrate rely on IM depot technology to support adherence in schizophrenia.

Important note

Intramuscular absorption is significantly reduced in patients with poor peripheral perfusion (shock, heart failure, hypotension). In these patients, IV is the only reliable parenteral route. The one notable exception: adrenaline in anaphylaxis is given IM into the anterolateral thigh — IV adrenaline carries a high risk of fatal arrhythmia and is reserved for specialist settings with cardiac monitoring.

Subcutaneous (SC)

Subcutaneous injection into the loose connective tissue beneath the skin provides slower, more sustained absorption than IM injection, because subcutaneous tissue is less vascularised than muscle. This makes it ideal for drugs where a steady, prolonged release is desired — insulin is the preeminent example. SC also supports self-administration with proper training, making it appropriate for outpatient chronic disease management (biologics such as adalimumab and etanercept, low-molecular-weight heparins).

Other parenteral routes

Intrathecal injection delivers drug directly into the cerebrospinal fluid, bypassing the blood-brain barrier — used for spinal anaesthesia (bupivacaine), intrathecal chemotherapy, and some severe spasticity treatments (baclofen pump). Intraosseous (IO) access into the medullary cavity of bone provides emergency vascular access when IV access is impossible — the onset and bioavailability are essentially equivalent to IV. Intradermal injection is used for allergy testing and BCG vaccine delivery.

Topical and transdermal routes

Topical application treats local tissue directly; transdermal delivery uses the skin as a controlled-release membrane for systemic drug absorption.

Topical

Topical drugs are intended primarily for local effect at the site of application — skin, eye (ophthalmic drops and ointments), ear (otic preparations), nose (nasal sprays for local decongestant effect), vagina, and rectum. Systemic absorption is usually unwanted but can occur, especially with large surface area application, broken skin, or occlusive dressings. Corticosteroid overuse on large areas of skin can lead to clinically significant systemic absorption and HPA-axis suppression.

Transdermal patches

Transdermal delivery systems (patches) are engineered to deliver drug through intact skin at a controlled rate for sustained systemic absorption. The stratum corneum is the rate-limiting barrier — only lipophilic, low-molecular-weight drugs penetrate it effectively. Once through, drug enters dermal capillaries and the systemic circulation, completely bypassing first-pass metabolism.

Successful transdermal drugs include nicotine patches, fentanyl patches (72-hour analgesia), buprenorphine, oestradiol and combined hormonal contraceptive patches, clonidine, and hyoscine (scopolamine) for motion sickness. The steady plasma concentrations achieved are particularly valuable for drugs with narrow therapeutic windows.

Clinical pearl

Transdermal fentanyl patches take 12–24 hours to reach steady-state plasma concentrations after application. They are emphatically not for acute pain — they are for patients with stable, chronic pain already tolerant to opioids.

Inhalation

The lungs offer an absorptive surface area of 70–80 m² and extremely thin alveolar membranes — making inhalation one of the fastest non-injection routes available.

Inhaled drugs can act locally in the airways (bronchodilators, corticosteroids in asthma and COPD) or be absorbed across the alveolar membrane for systemic effect. Gaseous and volatile anaesthetic agents reach the brain via this route within seconds. Inhaled insulin (Afrezza) has demonstrated rapid absorption kinetics useful for mealtime glucose management.

Particle size is the critical determinant of deposition site within the respiratory tract. Larger particles (5–10 µm) deposit in the oropharynx and upper airways; small particles (1–5 µm) reach the bronchioles and alveoli. This is why pressurised metered-dose inhaler (pMDI) technique and the use of spacers are clinically significant — poor technique results in most of the dose being swallowed rather than inhaled.

Clinical consideration

Inhaled corticosteroids (ICS) such as fluticasone and budesonide are primarily intended for local airway effect. However, the fraction swallowed and absorbed orally is subject to significant first-pass metabolism — which is intentionally exploited in the design of these molecules to minimise systemic side effects. Rinsing the mouth after ICS use reduces oropharyngeal candidiasis, but does not substantially alter systemic bioavailability.

Factors influencing route selection

Route selection is a multidimensional clinical decision. No single factor determines the choice — it is always a balance of priorities.

Drug-related factors

The physicochemical properties of the drug itself constrain the possible routes. Molecular size matters — large peptide drugs (insulin, biologics) are digested in the GI tract and must be given parenterally. Lipophilicity determines whether a drug can cross the stratum corneum for transdermal delivery or the blood-brain barrier for CNS action. Stability in the GI environment rules out oral administration for acid-labile drugs. Extent of first-pass metabolism may make oral bioavailability so low as to be clinically impractical.

Patient-related factors

The patient's clinical state often overrides all other considerations. An unconscious patient cannot take oral medications. A vomiting patient will not retain oral formulations. Patients with dysphagia, oesophageal disease, or GI malabsorption syndromes may need alternative routes. Paediatric patients may refuse injections, making oral or rectal routes preferable when clinically acceptable. Patients with needle phobia, or those who will be self-administering, favour non-injection routes.

Clinical urgency

When rapid effect is needed — anaphylaxis, status epilepticus, acute severe pain, cardiac arrest — oral administration is simply not fast enough. The IV route (or IM for adrenaline in anaphylaxis) becomes mandatory. Conversely, for chronic disease management where steady-state concentrations and long-term adherence are the goals, oral or transdermal routes are almost always preferred for their safety profile, convenience, and lower cost.

First-pass effect

No discussion of routes of administration is complete without understanding the first-pass (presystemic) effect — one of the most clinically significant phenomena in pharmacokinetics.

After oral absorption, drug is transported via the portal vein to the liver before reaching systemic circulation. Hepatic enzymes — predominantly the cytochrome P450 family — metabolise a proportion of the absorbed drug on this first transit. For some drugs, the extraction ratio is so high that oral bioavailability is drastically reduced, or the drug is converted to active metabolites that are themselves responsible for the pharmacological effect.

Key examples

High first-pass drugs: GTN (nearly 100% extracted orally), morphine (oral bioavailability ~30%), propranolol (~25%), lignocaine (essentially zero oral bioavailability). Prodrugs exploiting first-pass conversion: codeine → morphine (CYP2D6), enalapril → enalaprilat, clopidogrel → active thiol metabolite.

Routes that bypass the liver entirely — sublingual, buccal, transdermal, rectal (lower rectum), inhalation, and all injectable routes — avoid first-pass metabolism. This is why sublingual GTN achieves a rapid, potent effect that oral GTN cannot replicate. It is also why transdermal oestrogen delivers effective systemic concentrations at much lower doses than oral oestrogen formulations.

Pharmacogenomics alert

CYP2D6 genetic polymorphisms profoundly affect first-pass metabolism. Poor metabolisers on codeine receive minimal analgesia; ultra-rapid metabolisers produce dangerously high morphine concentrations. Knowing a patient's metaboliser status can be life-saving.

Comparative overview

Route	Onset	Bioavailability	First-pass avoided	Self-admin	Key use case

Knowledge checks

Test your understanding. These are the types of applied questions you will encounter in pharmacokinetics assessments and clinical practice.

Chapter summary

Routes of administration are the pharmacokinetic gateway that determines a drug's bioavailability, onset, and practical utility. The IV route offers complete bioavailability and fastest onset. Oral remains the most convenient for stable outpatients. First-pass metabolism is the key distinction between oral and non-oral routes. Transdermal and inhaled routes offer targeted or sustained delivery with specific physicochemical requirements. Clinical decisions on route selection must always integrate drug properties, patient state, and urgency of effect.

Routes of drug adminstration

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