BMS 113 · Common Core (Tronc commun)Section 4.5 · Routes of drug administration
Section 21 of 31
Choosing a route
Apply the factors that decide the route of administration to simple prescribing situations.
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Why this matters
Every earlier section described one route; this one shows how to choose between them for a real patient. A wrong choice can mean a drug that is vomited before it works, an injection that was never needed, or a delay that harms the patient. The five factors gathered here are the ones you will use in every prescribing situation later in the course.
What you need to know first
You should already know the main routes from sections 4.1 to 4.4 and be able to say roughly how quickly each one acts. It also helps to know that a patient who cannot swallow or who is unconscious needs special care.
Words to know (5)
Onset of action
The time between giving a medicine and the first sign that it is working. An intravenous drug has an onset of 30 to 60 seconds.
Default route
The route chosen first when nothing rules it out, which for most patients is the oral route. A stable patient who can swallow starts on the oral default.
Adherence
How closely a patient follows the agreed treatment, such as taking every dose. A patch taken once a week can improve adherence.
Depot injection
An injection that releases its medicine slowly over days or weeks. An oily intramuscular antibiotic can act as a depot.
Local effect
An effect that happens only at the place where the medicine was applied. Ear drops have a local effect in the ear canal.
One idea at a timeIdea 1 / 5
In one sentence Five factors decide the route: the patient, the drug, the cost, the disease and the effect wanted.
Five factors decide the route
The selection of the most appropriate route depends on several factors. The lecture notes list five: patient characteristics, drug factors, cost, the disease condition and the intended effect. Patient characteristics include age, consciousness, the ability to swallow, nausea or vomiting, pregnancy and the function of the liver and kidneys. Drug factors include solubility, stability in acid, the size of the dose, whether the drug is an irritant and how it is eliminated. Cost covers the price of the medicine, of the devices and of the staff time needed. The disease condition and the intended effect decide whether the drug must act at once, must act locally, or must act steadily for a long time. No single factor settles the question; the route is chosen where all five point in the same direction.
For example
A drug that is cheap and effective by mouth, in a patient who can swallow and is not in danger, satisfies all five factors through the oral route.
Pause and check yourself
List the five factors that decide the route.
Show the answer
Patient characteristics, drug factors, cost, the disease condition and the intended effect, and the route is chosen where all five agree.
Read detailed explanations
Intended effect: local or systemic decides the route
The single most useful question when choosing a route is not “how do I get the drug in?” but “where do I want it to act?”. If the answer is a surface — the skin, the conjunctiva, the ear canal, the nasal lining, the airways, the vagina or the rectum — then applying the drug there is usually the best answer, because it puts a high concentration exactly at the target while keeping the rest of the body away from it. That is why eczema is treated with a cream, conjunctivitis with eye drops, otitis externa with ear drops and vaginal infection with pessaries, and why inhaled drugs control airway disease at doses that would be inadequate if given systemically. If the answer is inside the body — a fever, an infection, a seizure, pain, high blood pressure — then the drug must reach the bloodstream, and the choice widens to the enteral and parenteral routes, weighed by speed, first-pass loss, patient condition and cost. Some surfaces then become doors rather than destinations: the nasal mucosa, the conjunctiva and the rectum all absorb enough drug to produce systemic effects, and transdermal patches exploit the skin deliberately. The key is to decide first whether the target is a surface or the blood, and to treat any absorption from a “local” site as an effect to be expected and monitored rather than a surprise. This one distinction explains most of the choices in this unit before any other factor is considered.
Cost, adherence and the least invasive route that works
Cost is listed as one of the five factors, and it is not a minor one. The price of a treatment includes the medicine itself, the device it is given with, the staff time and skill it demands, the monitoring it requires, and the consequences of getting it wrong. An intravenous antibiotic costs more than the same antibiotic swallowed not because the molecule is different, but because it arrives with a cannula, sterile fluids, a pump, nursing time and a risk of infection that may itself need treatment. A transdermal patch costs more than a tablet but may be cheaper than a course of injections, and it can improve adherence because it removes the need to remember several daily doses — imperfect adherence has its own cost in treatment failure and resistance. This is why the oral route is called the default: it is cheap to produce, cheap to store, cheap to give and cheap to correct when something goes wrong. Three practical habits follow. First, in a resource-limited setting, ask whether the oral route would work before accepting a more expensive one. Second, remember that the cheapest route that fails is the most expensive of all, so do not defend the oral route where it is genuinely unsuitable. Third, weigh the patient’s circumstances, not only the hospital’s: a daily injection may be impossible for someone who lives far from a clinic, while a depot injection or a patch that lasts days or weeks may be the only realistic option. Route choice is a clinical decision, but it is also an economic and social one, and both belong in the reasoning.
Explore the diagram
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How fast each route acts
The approximate delay before a drug acts, from 30 to 60 seconds after an intravenous injection to 30 to 90 minutes after a swallowed dose. The route is chosen to match the urgency and the patient.
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01 / 8Intravenous: 30–60 seconds
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Intravenous: 30–60 seconds
The fastest route, with an effect in 30 to 60 seconds. Drug is placed directly in the vein, so no absorption step is needed and bioavailability is complete. This is why intravenous injection is chosen in an emergency, but the dose must be given slowly and the patient watched closely.
Schematic timeline; not to scale. Milestones are evenly spaced for teaching, not positioned proportionally to time, and the quoted delays are approximate ranges that overlap in practice.
Key points to remember
✓Five factors decide the route: patient, drug, cost, disease and intended effect.
✓A patient who cannot swallow needs a non-oral route such as rectal or parenteral.
✓An emergency calls for intravenous, inhaled or sublingual administration.
✓Drug properties can rule out the oral route, for example acid destruction or irritation.
✓The oral route is the safe, cheap default whenever nothing rules it out.
Go deeper
Worked example
Applying the five factors to three patients. Three patients need the same antibiotic. Patient A is a stable adult who can swallow. Patient B is vomiting repeatedly. Patient C is unconscious after a road accident. You must choose a route for each and give the reason in terms of the factors listed in the notes.
Patient A: oral is convenient, cheap, safe and allows the widest choice of dosage forms, and the intended effect is a systemic one that does not require speed. No other factor overrides this.
Patient B: the oral route is excluded because the drug would be vomited before absorption. A rectal suppository is a reasonable alternative because it is absorbed through the rectal wall and partly bypasses the liver, or an intravenous dose can be used if the situation is urgent.
Patient C: an unconscious patient cannot swallow and cannot protect the airway, so the parenteral route is required; intravenous infusion gives an accurate dose, immediate action and effectively 100 % bioavailability.
Now weigh the remaining factors for all three: cost favours the oral tablet, the disease condition and intended effect favour speed in patient C, and drug factors would override everything if this antibiotic were destroyed by gastric acid or badly absorbed.
The same drug, the same diagnosis and the same prescriber can produce three different routes. The route follows the patient and the situation, not the medicine alone — and the reason for departing from the oral default should always be nameable.
Common misconception
Students sometimes believe that a more invasive route is a more serious or more effective treatment, so an injection must be better than a tablet. This is backwards: invasiveness buys speed or local delivery and pays for it in risk, cost and skill, so the oral route is preferred whenever it can do the job.
Check your reasoning
A prescriber writes an intramuscular injection for a patient who could take the same drug as a tablet. Using the factors from the notes, list the questions you would ask before accepting or challenging that choice.
Reveal an answer
Ask whether the patient can swallow and is not vomiting or unconscious; whether the disease needs an immediate effect; whether the drug is destroyed by acid, is poorly absorbed or has a large first-pass effect; whether it is too irritant for the stomach; whether a local effect is wanted; and whether a long steady effect is required. If none of these applies, the oral route is safer and cheaper, and the injection needs a stated reason.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Routes of administration, pp. 70–71
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About this course and its sources
Bilingual lessons organised from the supplied BMS 113 General Pharmacology I lecture notes. Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.