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Functional analogs (or functional analogues) are entities (models, representations or compounds) that can be replaced, to fulfill the same function. When the entities in question are formally represented by black boxes, the concept of analog is related to "same behaviour": they take the same output sequence when submitted to the same input sequence.
Analogical models are used in a method of representing a ‘target system’ by another, more understandable or analysable system. Two systems have analog functions (see illustration) if the black box representation of both can be the same.
Functional analogs are compounds that have similar physical, chemical, biochemical, or pharmacological properties. An example of a functional analog (and structural analog) is morphine and heroin. Functional analogs are not necessarily also structural analogs with a similar chemical structure. Examples are classes of drugs that have a similar mechanism of action.