On the basis of chemical structure

Classification by chemical structure groups together drugs that share a common structural skeleton or core framework, because molecules built on the same skeleton very often show similar pharmacological behaviour - the shared substructure is usually what is being recognised by the biological target.

The standard textbook example is the sulphonamide (sulpha) group of drugs. Every member of this family is built around a benzene ring that carries a $-SO_2NHR$ group and an $-NH_2$ group positioned para to each other on the ring. This shared skeleton is what gives every sulpha drug a broadly similar ant …