BMS 113 · Common Core (Tronc commun)Section 5.1 · Classification of drugs and major drug classes
Section 22 of 31
Ways of classifying drugs
Explain the main ways drugs are grouped for study, prescribing and dispensing.
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Why this matters
If you do not know how medicines are grouped, every new drug becomes a separate fact to memorise, and you may miss that two items on one prescription do the same job. This filing system is used again in every later section of this unit.
What you need to know first
You need only everyday knowledge of medicines as tablets, syrups or creams. Unit 2.6 explains the three names of a drug, which helps when one medicine appears under several labels.
Words to know (5)
Classification
The way we group medicines into families so that they can be learned, stored and used in an orderly way. Analgesics and antibiotics are two classifications of medicines.
Class (family)
A group of drugs that share something important, such as a chemical structure or the same effect. The penicillins form one class of antibiotics.
Basis of classification
The question used to make a group, for example what the drug does or how it is given. Grouping by route puts oral drugs in one list and injectable drugs in another.
Dosage form
The physical product that is dispensed, such as a tablet, a syrup, a cream or an injection. The same drug can exist as a tablet and as a syrup.
Route of administration
The path by which a medicine enters the body, such as swallowing, applying to the skin or injecting. An oral drug is swallowed, while a topical drug is applied to the skin.
One idea at a timeIdea 1 / 5
In one sentence Classification simply means putting medicines into groups so that they are easier to learn and use safely.
Classification is the way we group drugs
The lecture notes use five practical filing systems: chemical structure; mode of action or therapeutic effects; dosage forms; routes of administration; and the body system on which the drug acts. A class is not a law of nature. It lets a large and growing number of medicines be learned, remembered and handled safely in an orderly way, rather than memorised one by one as unrelated facts. Because the bases are independent, the same drug appears in several files at the same time.
For example
Paracetamol can be filed as an analgesic and antipyretic by effect, as a tablet or a syrup by dosage form, and as an oral drug by route of administration.
Think of it like
Think of a library: the same book can be found under its author, its subject or its title, and each catalogue answers a different question. A library catalogue, however, is fixed by librarians, while drug classes are practical inventions that can change as knowledge grows.
Pause and check yourself
Why do we group drugs instead of learning each one on its own?
Show the answer
Because there are far too many medicines to remember as isolated facts, and grouping them lets a learner study, store and handle them in an orderly way.
Read detailed explanations
Chemical structure: families that share a skeleton
Classification by chemical structure groups drugs that share a molecular skeleton or a characteristic chemical group. The penicillins, for example, share a common core structure, and amoxicillin and ampicillin are semi-synthetic modifications of penicillin G. This grouping is not academic, because relatives often behave alike. They tend to act by the same mechanism, to be handled by the body in similar ways, and to provoke the same allergy, which is why a patient allergic to one penicillin is treated cautiously with others of the family. Structure also predicts stability and route: a substance destroyed by stomach acid cannot be given orally, while a small fat-soluble molecule may cross the skin. When a new molecule is designed, chemists compare it with known structures to estimate its likely activity and toxicity before any patient receives it. The limitation is that structure does not guarantee effect. Two drugs may look alike yet act differently, and two drugs may act alike with completely different structures — aspirin and paracetamol both lower fever but are chemically unrelated. Structural classification is therefore a strong hint, never a proof, and it is always read together with the therapeutic and mechanism-based lists.
How medicines are arranged on the shelf
Classification becomes physical in the pharmacy. The notes describe medicines stored on shelves, in cupboards and in the refrigerator, always with their labels facing the dispenser so that the product can be identified quickly during dispensing. Heavy products are placed on the lower shelves and lighter ones higher, a simple safety rule that prevents injury and breakage. Within a shelf, drugs are arranged alphabetically using either the generic or the trade name, whichever suits the dispenser, and stock rotation follows FEFO — first to expire, first out — so that the product closest to expiry stands in front; when products share an expiry date, FIFO (first in, first out) applies, and LIFO (last in, first out) is used when the last product received expires soonest. Shelf-life, the period during which a formulation keeps its potency, is calculated as the expiry date minus the date of manufacture. The legal category also affects arrangement: a controlled substance is stored and recorded under stricter control than an ordinary over-the-counter product, and some drugs need cold storage. A tidy, correctly rotated shelf is not housekeeping; it is part of the safety system that keeps the wrong drug or an expired drug from reaching a patient.
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Five ways to classify a drug
One drug belongs to several families at the same time. Each of the five axes answers a different question for the student, the prescriber and the dispenser.
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01 / 7Chemical structure
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Chemical structure
Groups drugs by the molecule itself: compounds that share a chemical skeleton are studied together, so one family name covers many brand names. The penicillin family is the classic illustration, because its members are recognised from their common chemical nucleus rather than from what each one treats.
Schematic; not to scale. The five axes carry equal weight and the illustrations are teaching examples, not prescribing guidance.
Key points to remember
✓A class is a practical filing system, not a law of nature.
✓The notes give five bases: chemical structure, effect or mechanism, dosage form, route and body system.
✓One drug belongs to several groups at the same time because each basis asks a different question.
✓Classification helps the prescriber predict effects, the pharmacy dispense safely and the researcher compare.
Go deeper
Worked example
Classifying a medicine four ways. A pharmacy receives a new product: a 500 mg tablet of a medicine that lowers fever and pain, is swallowed with water, and acts on the body’s temperature regulation and pain signalling. The team must decide how to file it.
Therapeutic effect: it lowers fever and pain, so it is filed with analgesics and antipyretics.
Dosage form: it is a tablet, so it is filed with solid oral dosage forms.
Route of administration: it is swallowed, so it is classified as an oral medicine rather than a topical or injectable one.
Body system and mechanism: it acts on temperature regulation and pain signalling, so it is filed with drugs acting on the nervous system rather than, for example, the heart.
One product produced four correct answers because the four questions were different. Classification does not force a choice; it records each answer separately so that prescriber, pharmacy and researcher can each use the list they need.
Common misconception
Students often think a drug belongs to exactly one class and that finding “the” class is the goal. In fact the bases are independent, and the same drug is deliberately filed in several groups at once; a single label is correct only if you say which basis you are using.
Check your reasoning
Why does classifying drugs help a prescriber spot duplication of therapy?
Reveal an answer
Because drugs of the same class share the same therapeutic effect. When two medicines on a patient’s list belong to the same class, the prescriber can see that the second adds little benefit while adding the same family risks, and can remove one instead of adding a third.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Unit 5, pp. 19–21 and p. 50
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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.