: Do we have a general system of class of drugs that a layman can understand? When I was reading the accepted answer in the Why do doctors prescribe steroid tablets even though they know the side effects? question, I noticed

There is no need for any person to learn any drug taxonomy, unless this person is creating or extensively using clinical or pharmacological documentation. I don't doubt that somewhere, a single unified taxonomy of drugs exists, prescribed by some standardization body - and I also don't doubt that it's a major pain to use and as hated by physicians as ICD-10.

For all medical purposes outside of documentation, people use categorization the same way they do it for all other concepts in their life. They put a label on any group they (and hopefully their communication partner) readily recognize. And what they recognize depends on their level of expertise in the field.

This is why you, the patient, will naturally say "drugs for migraine" while talking to a friend of physician. A physician will use categories such as "serotonin receptor agonists". Neither of you two studied a taxonomy tree of drugs before using a correct category name. You knew "there are migraines" and derived a proper category name from it. The physician learned about the role of serotonin in the brain, and one chapter of his textbook explained how serotonin is connected to migraine, and another explained how there are drugs which mimic the effects of serotonin by activating the same receptors which are usually activated by serotonin, so they can stop a migraine.

Note that the two categories are not the same, even though they have some overlap. Some serotonin receptor agonists are a type of migraine drugs. But there are migraine drugs which are not serotonin receptor agonists, and there are serotonin receptor agonists which do something other than heal migraine, because they activate a slightly different set of serotonin receptors. But there is no simple way to translate the category "serotonin receptor agonists" to layman's terms. It's not even a matter of it resulting from too detailed criteria, and saying that it's good for a layman to learn the more general category above it - because the more general category of "receptor agonist" is not easier to understand. Experts' taxonomies are not simply more detailed versions of a layman's taxonomy of the same area, they are orthogonal to laymen taxonomies, because they are based on completely different principles.

If what you want is a categorization system reflecting your current knowledge, then you already have it. Talking about "drugs for fever" or "drugs for migraine" is not wrong in any way. Just say whatever you mean when you need it. There could be a category which you need to talk about but cannot come up with a succinct name for it, such as "drugs for fever which are safe for toddlers and are taken through the mouth" - but it is unlikely that it already makes a node in somebody else's standardized taxonomy, you have to describe it. And while your doctor might be inclined to use the description "pediatric oral antipyretic" instead, this does not make your label less valid.

If you want to be able to work with a more intricate taxonomy, you'll need more medical knowledge. And that's great - everybody can use knowledge about such an important subject. But in this case, the knowledge comes first. Using the correct taxonomy will come naturally, as a byproduct of your increased knowledge. The other way round does not work.

Of course, it can happen that you come across information which refers to a category you don't understand, for example in the list of interactions of a drug you are taking. But if you want to understand this information, the way is not through some special taxonomy (which cannot be mapped to concepts you already know anyway). If you really need to know what makes a class of drug a "class" and not a random collection of drugs, and a source you find does not explain it in terms you understand, you need another source which will try to explain the criteria behind the existing expert's category. I'm sure the users of this site will be happy to help you in this, too.

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I second rumstscho's answer (except the part about ICD-10 being a major pain to use).

There is a classification system of medicines that is quite detailed and widely used, and it is called:

Anatomical Therapeutic Chemical (ATC) classification system

As you can guess from the name it classifies medicines based on:

the organ or system on which they act and
their therapeutic properties and
pharmacological properties and
chemical

How do they take all of these into account? By using a multilevel classification system. The classification is paired with a code system, which can be used to search for a medicine on a regulatory agency's website, for instance.

ATC classification system has five levels:

First level - anatomical main group (this is a level a layperson can easily understand)

A Alimentary tract and metabolism

B Blood and blood forming organs

C Cardiovascular system

D Dermatologicals

G Genito-urinary system and sex hormones

H Systemic hormonal preparations, excluding sex hormones and insulins

J Antiinfectives for systemic use

L Antineoplastic and immunomodulating agents

M Musculo-skeletal system

N Nervous system

P Antiparasitic products, insecticides and repellents

R Respiratory system

S Sensory organs

V Various

Second level - therapeutic main group (this is a level that an informed patient can understand - if you are somewhat familiar with the medical condition/indication the medicine is for, you can understand this level)

Third level - therapeutic/pharmacological subgroup (this is where things get quite technical; these waters are generally too deep for a layperson)

Fourth level - chemical/therapeutic/pharmacological subgroup

Fifth level - the chemical substance

How are medicines included in the system:

Inclusion and exclusion criteria
The WHO Collaborating Centre in Oslo establishes new entries in the ATC classification on requests from the users of the system. These include manufacturers, regulatory agencies and researchers. The coverage of the system is not comprehensive. A major reason why a substance is not included is that no request has been received. [...]

Complementary, homeopathic and herbal traditional medicinal products are in general not included in the ATC system.

from: WHO Collaborating Centre for Drug Statistics Methodology

(if you are really interested in this subject, you might find some chapters of this publication interesting.

However, there is no need for you to learn this (or any other classification). If your goal is to gain knowledge on medicines the area you are interested in is pharmacology or more precisely pharmacodynamics. This is quite a large area, but a real catch for a layperson is that it is an applied science, sou you would need knowledge from physiology, patophysiology and medicinal biochemistry first; and for those you would need cell biology, some anatomy and histology, microbiology, biochemistry (for which you definitely need some chemistry)... This would be a few year's quest and you would still need a curriculum and someone to supervise your learning process to make sure you understand all important concepts correctly.

This doesn't mean that you can't be a well informed, educated patient (or patient's caregiver, family member). You just don't need to learn about all of the major illnesses and medicines. Simply, when (if) a health problem occurs focus your efforts on that specific area. You cannot and should not use the knowledge you gain to self-medicate; it should serve you to communicate better with your health care providers, participate in the decisions, and if necessary consider if it's time to get a second opinion on something.

An aside: Here is an example of how a book in pharmacology is organised. The lessons about specific medicines start from section 2. You can see that sometimes a cellular/chemical mechanism is used (section 2), and sometimes a whole organ or system of organs (section 3 and 4) or the disease to be treated (section 5 and chapters 43-45 in section 4 e.g.) (whichever is better to explain how a certain medicine works). I do not recommend this book for you (not that it's not good, it's a great one) - because it's designed for grad students of medicine/pharmacy as well as phd students. While it is great because it encourages critical thinking, you can get lost in the quantity of details. I've just used its table of contents as an example how one can go about studying pharmacology. For a layperson I'd say that starting with Wikipedia is not a bad thing (articles there are usually well organised), as long as you make sure to check the accuracy of information you find there.

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