BMS 113 · Common Core (Tronc commun)Section 6.4 · Prescription writing, dosage and safe medicine use
Section 29 of 31
Indications, contraindications and the therapeutic window
Explain what makes a drug appropriate, inappropriate or unsafe for a particular patient, using indication, contraindication, caution and the therapeutic window.
New to this course? How each section works
Learn gives you one short idea at a time. Each idea starts with a single sentence, then explains it, gives an example, and asks you a small question you can answer straight away.
Explore shows a diagram of the same ideas. Click the numbered parts to read what each one is.
Check asks a few questions. Answer them all correctly to finish the section and unlock the next one.
“Go deeper” is optional. It adds mechanism and extra detail for when you are ready — nothing you need for the checks is hidden there.
Why this matters
If you treat every drug as simply allowed or forbidden, you will either withhold a useful medicine from a patient who could take it safely with monitoring, or give one to a patient who should not receive it. In practice that is the difference between a treated patient and a harmed one. You will use these filters again in Unit 6.5, when you study the errors that reach patients, and in Unit 5.4, where suspected adverse effects are reported.
What you need to know first
You need only the everyday idea that a medicine is chosen for a particular problem, and that people differ in age, pregnancy and organ health. Unit 5.2 explains prescription-only and over-the-counter drugs, and this section builds directly on that distinction.
Words to know (6)
Indication
The reason to give a drug: the condition, symptom or risk that the drug is expected to help. A bacterial infection is an indication for an antibiotic.
Contraindication
A situation in which a drug should not be given because the expected harm outweighs any likely benefit. A known allergy to the drug or to its class.
Absolute and relative contraindication
An absolute contraindication means the drug must not be used at all; a relative one means it is avoided unless the need is compelling and no better option exists. Severe allergy is absolute; pregnancy is sometimes relative.
Caution (precaution)
A situation in which the drug may still be used, but only with extra care such as a reduced dose, a slower start or closer monitoring. Reduced kidney function usually means adjust and watch.
Therapeutic window
The gap between the maximum effective concentration of a drug and the minimum lethal concentration. Digoxin has a narrow window and is monitored.
Risk–benefit balance
The comparison of the benefit a drug is expected to give against the harm it may cause, judged for one particular patient. The same drug may be justified in severe illness and unjustified for a mild symptom.
One idea at a timeIdea 1 / 6
In one sentence An indication is the reason to give a drug: the condition, symptom or risk that it is expected to help.
Indication: the reason to give the drug
An indication is the reason to give a drug: the condition, symptom or risk that the drug is expected to help. A medicine without an indication is a medicine without a purpose, and even a powerful drug is the wrong treatment if the problem it addresses is not the problem the patient has. Indications are established during drug development and then refined in practice, where they describe the patients in whom benefit has been demonstrated. Writing an indication forces the prescriber to state the clinical target — an infection, a raised blood pressure, a pain — before choosing a product. When the indication is unclear, the safest next step is usually to clarify the diagnosis rather than to add a medicine.
For example
An antibiotic has an indication only when a bacterial infection is likely or proven; given for a viral cold it adds risk without benefit.
Think of it like
An indication is like the destination written on a ticket, but with one difference: it is not fixed in advance, because it is a clinical judgement that must be made again for each patient.
Pause and check yourself
What is an indication, and what does it mean when there is none?
Show the answer
It is the condition the drug is meant to help. Without an indication the medicine has no purpose, and the safest next step is usually to clarify the diagnosis rather than to add a drug.
Read detailed explanations
The therapeutic window measured in the blood
The definition in the notes compares two concentrations, not two doses: the maximum effective concentration and the minimum lethal concentration. That wording contains the central idea of clinical pharmacology. A drug acts according to the concentration it reaches at its target, and the same dose can produce different concentrations in different patients, because absorption, distribution, metabolism and excretion all vary with age, weight, food, other drugs and organ function. A wide window means that this natural variation stays comfortably inside the safe range. A narrow window means that the same variation can push a patient above the therapeutic range, where the effect becomes toxicity, or below it, where the treatment simply fails. This is why narrow-window drugs such as digoxin are both prescription-only and monitored: the prescriber is not protecting against a careless patient but against a range of concentrations that is intrinsically easy to cross. Monitoring makes the invisible visible by measuring the blood level, and it allows the dose to be adjusted to the individual rather than to the average. The same reasoning explains why two patients on the same prescription can respond differently without either of them being “wrong”, and why a relapse or a side effect should prompt a question about concentration before a conclusion about the drug or the patient.
Why benefit–harm is patient-specific
The categories of contraindication and caution look like a list to memorise, but each one is a mechanism reaching the same conclusion by a different route. Allergy is an immune mechanism: the first exposure created antibodies or sensitised cells, and the second exposure can produce a reaction out of proportion to the dose. Pregnancy and breastfeeding are distribution mechanisms: a drug that stays in the mother may cross the placenta or enter milk, exposing an organism whose liver and kidneys are not yet able to clear it. The extremes of age combine composition and clearance: a child’s smaller body and immature organs change the dose that reaches the target, and an older person’s reduced kidney and liver function, lower body water and multiple medicines change both the concentration and the number of interactions. Kidney and liver failure are accumulation mechanisms: when elimination or metabolism slows, a normal dose produces an abnormal concentration. Interactions are competition mechanisms: one drug induces, inhibits or displaces another, changing how much of it is active. Disease-specific cautions are vulnerability mechanisms: an organ already damaged, or a system already unstable, tolerates less insult. Seen this way, the list stops being arbitrary and becomes derivable. A prescriber who understands the mechanisms can reason about a patient who does not appear in any list, which is exactly what safe prescribing requires.
Explore the diagram
Look at the whole drawing first. Then select one numbered part and connect it to the idea you just read.
Indication, contraindication and caution
A decision layout for indication, contraindication and caution, beside a therapeutic-window panel and the risk–benefit balance that governs prescribing.
New to diagrams? Read this first
Look at the whole drawing once without clicking anything. Ask yourself only: what kind of picture is this (a comparison, a route, a sequence, a table)?
Press Play, or use the arrows, to move through the numbered parts in order. The drawing stays still; only the highlight moves.
Click a number, or its name in the list below, to read what that part is and why it matters. Use “Hide labels” when you want to test your memory.
More controls
01 / 5Indication
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Indication
The indication is the reason to give the drug — the condition the medicine is expected to help. It rests on a clear diagnosis, because an effective drug given for the wrong condition is still the wrong treatment. Stating the indication keeps prescribing tied to a clinical need rather than to habit or to a patient’s request.
Schematic; not to scale. The zones overlap in practice: a relative contraindication may become a caution, and a caution may become a contraindication as the patient changes.
Key points to remember
✓An indication is the reason to give the drug; without one, a medicine has no purpose.
✓A contraindication says do not give the drug; a caution says give it only with a plan.
✓Contraindications and cautions come from a small set of reasons: allergy, pregnancy and breastfeeding, age, kidney or liver function, interactions and specific diseases.
✓The therapeutic window is the gap between effective and lethal concentrations, and a narrow window needs supervision.
✓Prescribing weighs expected benefit against expected harm for this patient, and gives a drug only when it is justified.
Go deeper
Worked example
Deciding whether a medicine suits one patient. A patient with joint pain also has a history of peptic ulcer disease, reduced kidney function and a known allergy to one antibiotic. A prescriber is considering a painkiller of the anti-inflammatory (NSAID) type. The team must separate the questions that decide the answer.
Indication: what is the drug expected to help? If the pain is mild and self-limiting, the indication may be too weak to justify a drug with significant risk.
Contraindication: the history of peptic ulcer disease is a reason not to give an NSAID-type anti-inflammatory, because this family can harm the stomach lining and aggravate an existing ulcer.
Caution: the reduced kidney function may still allow some drugs, but it requires a plan — a different choice, a reduced exposure or closer monitoring.
Allergy: a known allergy to a drug or to its class removes that medicine from consideration entirely.
Risk–benefit: only after these filters does the prescriber weigh what the drug is likely to achieve against the harm it may cause, and consider whether a safer option or no medicine at all is preferable.
An appropriate drug is not the most effective drug in general; it is the one whose indication is real, whose contraindications are absent, whose cautions are managed, and whose benefit clearly outweighs its harm for this patient.
Common misconception
Beginners tend to treat a contraindication as a warning to be careful and a caution as a rule to be obeyed. It is the reverse: a contraindication says do not give the drug, while a caution says give it only with a plan.
Check your reasoning
Why can the same drug be clearly justified in one patient and contraindicated in another?
Reveal an answer
Because benefit and harm are patient-specific. The drug does not change, but age, pregnancy, kidney and liver function, allergies, other medicines and the severity of the illness change both what the drug can achieve and what it can damage, so the balance — and sometimes the whole decision — changes with the patient.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Clinical therapeutics outline: indications and contraindications, course outline p. 2
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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.