BMS 113 · Common Core (Tronc commun)Section 4.1 · Routes of drug administration
Section 17 of 31
Enteral routes
Explain buccal, sublingual, oral and rectal administration and their first-pass differences.
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Why this matters
How a medicine enters the body changes how fast it works, how strong it is and how safe it is. If you ignore the route, you may give a tablet to a patient who is vomiting, or expect a swallowed drug to act within minutes. These ideas return in sections 4.2 to 4.5, where the same questions decide every route.
What you need to know first
You should already know that a medicine is a chemical substance that changes how the body works, and that the body has a digestive tract and a bloodstream. Unit 3.5 describes the dosage forms used for injections and other routes.
Words to know (5)
Route of administration
The path by which a medicine is brought into contact with the body, such as swallowing or injecting. Swallowing a tablet is the oral route.
Enteral
Given through the gastrointestinal tract, from the mouth to the rectum. A swallowed tablet and a rectal suppository are both enteral.
First-pass effect
The part of a drug that the gut wall and liver chemically change before it reaches the general circulation. Nitroglycerin is destroyed this way, so it is placed under the tongue.
Bioavailability
The fraction of the dose that reaches the general circulation unchanged and can therefore act. An intravenous dose is almost 100 per cent bioavailable.
Sublingual
Placed under the tongue, where the drug dissolves and is absorbed without being swallowed. A nitroglycerin tablet for chest pain is taken sublingually.
One idea at a timeIdea 1 / 6
In one sentence A route of administration is simply the way a medicine gets into or onto the body, such as swallowing, injecting or applying it.
A route is the path a substance takes into the body
The route matters because dosage forms do not all have the same possible paths: some drugs are formulated for several routes, while others are limited to one. Administration supports diagnosis, prevention and treatment, but the appropriate path depends on the patient, the drug and the intended effect — factors developed later in this unit.
For example
Paracetamol can be swallowed as a tablet, given as a syrup to a child who cannot swallow tablets, or inserted as a suppository when the patient is vomiting; insulin, a protein, is destroyed in the gut and therefore cannot be given orally at all.
Think of it like
Think of a route as a delivery road into a walled city: the same parcel can arrive by different roads, and each road has its own gate, speed and tolls. The comparison stops at the wall, because the body also changes the parcel on the way in.
Pause and check yourself
In one sentence, what is a route of administration?
Show the answer
It is the path a medicine takes to reach the body, for example swallowing a tablet or injecting into a muscle.
Read detailed explanations
First-pass metabolism: why the same dose behaves differently
When a drug is swallowed, it is not delivered to the body in one step. It dissolves in the stomach and intestine, crosses the cells of the gut wall, and enters the portal vein, which carries it straight to the liver before it can reach the heart and the general circulation. The gut wall and the liver contain enzymes whose normal job is to break down foreign chemicals, and they treat the drug the same way. Part of the dose is chemically changed — usually into less active products — during this first passage. The fraction that survives to reach the general circulation unchanged is the bioavailability of that route. This single mechanism explains several everyday facts. It explains why an oral dose is often much larger than an intravenous dose of the same drug: the prescriber is compensating for what the gut and liver will remove. It explains why some drugs cannot be given orally at all, such as nitroglycerin, which is almost completely destroyed on its first pass and is therefore placed under the tongue, where the veins drain into the general circulation and skip the liver. It also explains why the rectal route sits between the two: the lower rectum drains partly into veins that bypass the liver, but the upper rectum drains into the portal system, so absorption there is variable — sometimes largely protected from first pass, sometimes not. Finally, it warns against judging a dose by its number alone: 500 mg swallowed and 500 mg injected are not the same amount reaching the tissues.
Disintegration and dissolution: why a swallowed tablet is not instant
A tablet is not a drug; it is a drug mixed with excipients and compressed into a shape. Before any absorption can happen, two physical steps must occur. Disintegration is the tablet breaking apart into granules and small particles in the fluids of the gut. Dissolution is those particles dissolving into the fluid so that individual drug molecules are free to cross the intestinal wall. Only dissolved molecules can be absorbed. This sequence explains the slow onset of action of solid oral forms compared with liquids, and it explains why a syrup or a solution acts sooner than a tablet of the same drug. It also explains why absorption can be incomplete: a tablet that passes through the gut before it has fully dissolved releases only part of its dose, and a drug that dissolves poorly in water will always be absorbed erratically. Food adds another layer. A meal changes the speed at which the stomach empties, dilutes the drug, and can bind it chemically — calcium in milk forms an insoluble complex with tetracycline, so the antibiotic is carried out in the stool instead of being absorbed. Conversely, some drugs dissolve better with food or a fatty meal, which is why propranolol is absorbed well with food and griseofulvin needs a fatty meal. The practical consequences for a beginner are simple: read whether a medicine should be taken with or without food, never crush an enteric-coated or slow-release tablet without advice, and expect a liquid form to act more quickly than a solid one.
Explore the diagram
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Enteral routes and first pass
Enteral doses enter through the gut. Buccal, sublingual and rectal doses partly avoid the liver, which is why they survive the first pass better than a swallowed tablet.
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01 / 7Buccal
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Buccal
Drug is held between the gums and the teeth and dissolves slowly. The mouth has a rich blood supply, so absorption is rapid, food does not interfere, and the neutral pH of the mouth keeps some drugs more stable than in the stomach. Only small doses and non-irritant drugs suit this route.
Schematic; not to scale. Vein paths are simplified: in reality only part of the rectal drainage bypasses the liver, and the fraction of a dose escaping first pass varies from drug to drug.
Key points to remember
✓A route is simply the path a medicine takes into the body.
✓The enteral routes are buccal, sublingual, oral and rectal.
✓Buccal and sublingual drugs dissolve in the mouth and avoid the first pass through the liver.
✓The oral route is convenient, cheap and safe, but slow and unsuitable for vomiting or unconscious patients.
✓The rectal route partly avoids the liver and suits patients who cannot swallow.
Go deeper
Worked example
Choosing between four enteral routes for one patient. A 68-year-old man arrives with crushing chest pain and is also known to have a fever and oral thrush. He is fully conscious but nauseated. You are asked which enteral route to use for the nitroglycerin, which for the antipyretic and which for the antifungal, and why.
For the chest pain, choose the sublingual route: the tablet dissolves under the tongue, is absorbed into the rich blood supply beneath it, and acts within about two minutes — the reduction in first-pass elimination and the neutral pH of the mouth both help.
For the fever, the oral route is convenient and safe in a conscious patient, but he is nauseated, so keep the rectal route ready: a suppository is absorbed through the thin, richly supplied rectal wall and part of its blood drains directly to the general circulation, bypassing the liver.
For the oral thrush, use the buccal forms — Daktarin buccal gel or clotrimazole (Candid) buccal solution — because here the aim is a local effect on the mouth lining, and keeping the drug in the mouth is exactly what is wanted.
Finally, check the limits: the sublingual route holds only a small dose and is inconvenient for frequent use, so it is kept for the emergency dose rather than for long-term treatment.
One patient can need several enteral routes at once. The choice follows the intended effect (local or systemic), the speed needed, the patient’s ability to swallow, and the first-pass fate of the drug.
Common misconception
A common beginner error is to treat every medicine taken by mouth as “oral”, and to assume a sublingual tablet is simply a small tablet to swallow. Sublingual and buccal drugs work precisely because they are dissolved in the mouth and not swallowed; swallowing them destroys the speed and the first-pass advantage that the prescriber intended.
Check your reasoning
The same drug is available as a swallowed tablet and as a suppository. In which patient, and for which reason, would the suppository give a higher amount of drug in the bloodstream?
Reveal an answer
In a patient who is vomiting, or whose gut and liver would destroy much of the swallowed dose. The suppository avoids the stomach and part of the first-pass effect, because some veins draining the rectum reach the general circulation without passing through the liver; the swallowed tablet may never be absorbed at all if it is vomited.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Routes of administration, pp. 59–63
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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.