Drugs can be classified according to various criteria. Usually, they are classified according to
a) chemical structure,
b) pharmacological action,
c) therapeutic use,
d) anatomic therapeutic chemical structure (ATC), and
e) mechanism of action at the molecular level.
The first classification was used extensively some years ago and is still preferred by some authors. Following this classification, drugs fall in one or more of these categories: amides, acids, alcohols, amino ketones, amino acids, enols, esters, lactones, sulfones, phenols, etc.Unfortunately, classifying drugs according to their chemical structural type has the disadvantage that members of the same structural group often exhibit very different types of pharmacological activity. Steroids, for example, may act as hormones (testosterone), diuretics (spironolactone).
Pharmacological classification takes into consideration the mode of action of drugs. According to this criterion, WHO divided the known drugs into the following classes: Central Nervous system depressants, central nervous system stimulants, peripheral nervous system drugs, smooth muscle active drugs, drugs acting at synaptic and neuroeffector junction sites, histamine and anihistamines, cardiovascular drugs, gastrointestinal tract drugs and others.
Each class is subdivided. For example, central nervous system depressants include the following groups:
a) general anesthetics,
c) central voluntary muscle tone modifying drugs,
d) analgesics, and
e) antivertigo drugs.
Classification following the third criterion, therapeutic use, is very similar to the pharmacological classification and in many cases, identical with it. Thus, drugs are divided into the following classes: central nervous system depressants, central nervous system stimulants, psychotropic drugs, peripheral nervous system drugs, muscle relaxants, spasmolytics, vitamins, anti-infectives, and other therapeutic uses. Each class is subdivided.
Thus anti-infectives comprise
a) antiprotozoal drugs,
d) antituberculosis drugs,
e) urinary tract chemotherapeutics,
f) systemic antibiotics, etc.
Anatomic therapeutic chemical (ATC) classification is adopted in the Nordic countries (Denmark, Finland, Iceland, Sweden). This very useful system for clinicians, divides the drugs into 14 main â€œanatomic groups (first level), distinguished by letters:
A, alimentary tract and metabolism; B, blood and blood-forming organs; C, cardiovascular system; D, dermatologicals; G, genitourinary system and sex hormones; H, systemic hormonal preparations, excluding sex hormones; J, general anti-infectives, systemic; L, antineoplastic and immunosuppressive drugs; M, musculoskeletal system; N, central nervous system; P, antiparasitic products; R, respiratory system; S, sensory organs; and V, various.
Each main group is further divided into four subgroups: the second level â€œtherapeutic main groupâ€�, distinguished by two digits; the third level, â€œtherapeutic subgroupâ€�, distinguished by a letter; the fourth level, â€œchemical/therapeutic subgroupsâ€�, distinguished by a letter; the fifth level, â€œchemical subgroupâ€�, distinguished by a two-digit number specific for each substance. For example, the complete classification of diazepam preparations is ATC = NO5BAO1: N central nervous system 05 psycholeptics B tranquilizers A benzodiazepine derivates 01 diazepam
Attempts to classify drugs according to their mechanism of action at the molecular level cannot include all drugs, because for many of them such a mechanism is yet unknown. For the present it is possible to divide a great number of drugs into the following classes: drugs acting on enzymes, drugs acting as suppressors of gene function, drugs acting by metabolic antagonism, drugs acting on biological membranes, and drugs acting by their psychochemical properties.
Drugs acting on enzymes can be subdivided into the following groups: a) enzyme activators: for example, of adenylate cyclase; b) enzyme inhibitors: for example, of acetylcholinesterase, amine oxidase, c) enzyme reactivators: for example, of acetypcholinesterase inactivated by organophosphates.