On the basis of drug action

Classification by drug action sorts drugs according to the specific biochemical process or molecular event that the drug interferes with, which is a narrower and more mechanistic criterion than overall pharmacological effect.

A drug's action can be identical across very different conditions. All antihistamines, for example, are grouped together because every member of the class blocks the biochemical action of the compound histamine (the mediator responsible for inflammation) - regardless of whether the particular antihistamine is being prescribed for an allergic reaction, a cold, or a stomach-acid problem. This is why the same underlying action can appear under different pharmacological-effect labels …