BMS 113 · Common Core (Tronc commun)Section 4.2 · Routes of drug administration
Section 18 of 31
Parenteral routes
Describe each parenteral route, its volume limits and its risks.
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Why this matters
Injections are powerful because they put a drug straight into a tissue or the blood, but each tissue accepts only a small volume and carries its own risks. Confusing the routes can cause pain, infection or a drug that never reaches the blood. These ideas return in section 4.5, where speed, patient condition and cost decide the route.
What you need to know first
You should already know that the skin has layers and that blood flows through vessels, and that a medicine can be measured in millilitres. Unit 4.1 describes the oral route, which parenteral routes avoid.
Words to know (6)
Parenteral
Given by injection through the skin rather than through the digestive tract. A vaccine given into the arm muscle is parenteral.
Intradermal
Injected into the dermis, the thin layer of the skin just below the surface. A tuberculin test uses 0.1 ml intradermally.
Subcutaneous
Injected into the fatty tissue just under the skin. Insulin is often injected subcutaneously.
Intramuscular
Injected into a muscle, which has a rich blood supply. Many vaccines are given into the deltoid muscle.
Intravenous
Injected or infused directly into a vein, so the whole dose enters the blood. An emergency antibiotic is often given intravenously.
Aseptic
Free of germs, using a technique that prevents infection during an injection. Washing hands and cleaning the skin keeps the procedure aseptic.
One idea at a timeIdea 1 / 6
In one sentence Parenteral means giving a medicine by injection through the skin, so it never passes through the digestive tract.
Parenteral means injecting through the skin
In parenteral administration a drug is injected through a hollow needle into the body at various sites and depths. This definition gives the whole family its name: the route bypasses the gastrointestinal tract, so the drug enters the tissues or the blood directly. The major parenteral routes are intradermal (ID, into the skin), subcutaneous (SC, under the skin), intramuscular (IM, into muscles), intravenous (IV, into veins), intra-arterial (IA, into arteries), intrathecal (IT, into the cerebrospinal fluid), intraperitoneal (IP, into the peritoneal cavity) and intra-articular (into synovial fluid). All parenteral dosage forms are sterile preparations, packed in single-dose vials or multi-dose ampoules or in plastic or glass containers; they are prescription-only drugs and are administered aseptically by competent health care personnel only.
For example
A vaccine can be given into the skin, under the skin or into a muscle, while insulin is usually given under the skin — three different needle depths for one word, “injection”.
Think of it like
An injection is like posting a letter through a side door instead of the main sorting office. The comparison has a limit: unlike a letter, the drug can be contaminated as it enters, which is why sterility matters.
Pause and check yourself
What does parenteral mean in ordinary words?
Show the answer
It means the drug is injected through the skin instead of being swallowed, so it enters the tissues or the blood directly.
Read detailed explanations
Why each tissue accepts only so much volume
The volume limits of the parenteral routes are not arbitrary numbers to memorise; they follow from the structure of the tissue. The dermis is a thin, dense layer bound above by the epidermis and below by the subcutaneous fat. It has little room to expand, and its many small vessels make it an efficient absorbing surface, so intradermal injections use only about 0.1 to 0.2 ml; a larger volume would produce a painful, blanched swelling and leak back out. The subcutaneous layer is loose fatty tissue with few large vessels but a working lymphatic drainage. It can hold about 0.5 to 1 ml comfortably and releases drug slowly, which is why it suits protein drugs, suspensions and slow-release implants. Muscle is a thick, elastic tissue with a dense capillary network, so it can accept roughly 1 to 3 ml and still absorb rapidly; its rich blood flow also means that an oily depot placed there is taken up steadily over a long period. A vein has no capacity limit at all in the same sense — the blood volume itself is the reservoir — so large volumes can be infused, but the drug cannot be retrieved once injected, which is exactly why the dose must be correct and the injection given slowly. Two practical rules follow. First, never force a volume into a tissue that was not designed for it. Second, when a patient is thin, oedematous or has poor perfusion, the same volume behaves differently, so the site and technique must be reassessed rather than repeated by habit.
Asepsis, embolism and the price of the parenteral route
Every parenteral route breaks the skin, the body’s first defence, and deposits drug in a space that is normally sterile or in direct contact with the blood. That is why all parenteral dosage forms must be sterile preparations, packed in single-dose vials or multi-dose ampoules or in plastic or glass containers, why they are prescription-only, and why they may be administered only aseptically by competent health care personnel. The risks follow from the same fact. Infection can be introduced by the needle, by contaminated solution, or by poor skin preparation; a multi-dose ampoule that is reused carelessly is a classic source of contamination. Embolism is the entry of air or of particulate matter into the circulation, which can block a vessel; this is why intravenous lines are cleared of air bubbles and why poorly soluble substances and oily solutions are not given intravenously. Tissue damage occurs when a drug that belongs in a vein leaks into surrounding tissue, or when a strongly irritant solution is placed in muscle. Speed itself is a risk: because an intravenous dose cannot be recalled once given, the injection must be given slowly and the patient closely monitored, and drugs with a narrow margin are the most dangerous in this respect. Notice the trade-off that runs through this whole unit. The parenteral routes offer speed, precision and freedom from the gut, and they charge for it in skill, cost, pain and risk. That balance, not a preference for injections, is what should decide the route.
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Parenteral routes by depth
A hollow needle places drug at a chosen depth: in the dermis, in fatty tissue, in muscle or directly in a vein. Depth decides volume, speed of absorption and risk.
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01 / 7Intradermal
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Intradermal
Injected into the dermis, which holds more blood vessels than the deeper subcutaneous layer. Volumes are very small, usually 0.1 to 0.2 ml. The route is used for allergy and disease screening, for local anaesthetic and for vaccines, and a positive skin reaction is read directly at the site.
Schematic cross-section; not to scale. Layer thicknesses, needle angles and depths are drawn for teaching and do not represent true anatomy or injection geometry.
Key points to remember
✓Parenteral means injected through the skin, bypassing the digestive tract.
✓Each route reaches a different depth: intradermal, subcutaneous, intramuscular and intravenous.
✓The volume a route accepts is set by the tissue, from 0.1 ml in the dermis to large volumes in a vein.
✓Muscle and veins absorb fastest because they are richly supplied with blood.
✓Every parenteral act must be sterile and performed aseptically.
Go deeper
Worked example
Matching needle depth and volume to the patient. A clinic must give three different medicines today: a tuberculin skin test, a vaccine for a child, and a rapid-acting antibiotic for a patient who is vomiting. The available information is the route list, the usual volumes and the risks. You must justify each choice.
The tuberculin test goes intradermally: a tiny volume of 0.1 to 0.2 ml is placed in the dermis, which has more blood vessels than the subcutaneous layer, and the local reaction is what will be read.
The vaccine can go subcutaneously or intramuscularly according to the product: the subcutaneous space holds 0.5 to 1 ml and absorbs through capillaries and lymphatics, while muscle holds 1 to 3 ml and absorbs faster because it is richly supplied with blood.
For the vomiting patient, the oral route is excluded, so choose intravenous: the whole dose enters the vein, bioavailability is effectively 100 %, and there is no first-pass metabolism or gastric irritation.
Then check the safety conditions of every parenteral act: the product must be sterile, given aseptically by competent personnel, injected slowly where required, and the patient monitored afterwards.
Parenteral routes differ by tissue, depth, volume and speed. Choosing well means matching the needle to the tissue, the volume to the space, and the speed to the clinical need — while never losing sight of sterility.
Common misconception
Beginners often assume that every injection works the same way and that “more direct” always means “faster”. The routes are not interchangeable: 0.1 ml cannot be given intramuscularly, an oily solution cannot be given intravenously, and an intradermal injection is read as a local reaction rather than for a systemic effect.
Check your reasoning
Intravenous administration is described as 100 % bioavailable, yet it is not automatically the best route for every patient. Give two reasons a prescriber might still choose another route.
Reveal an answer
First, its risks and costs: high infection risk, embolism, pain, the need for expert skill, special devices and aseptic procedures. Second, its short duration of action, which makes it unsuitable when a long steady effect is wanted — a subcutaneous implant or an intramuscular oily depot can deliver the drug for weeks, whereas an intravenous dose is gone quickly.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Routes of administration, pp. 64–66
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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.