Chemicals

The classic Dr. Jekyll and Mr. Hyde story involves a potion or chemical that splits a man into his Id and Ego.

The concept is still in use today. Chemical sprays that are shot at unsuspecting individuals, poisons to turn Emperor’s into a sex . Typically these chemicals are comical forms of estrogen, testosterone, or extracts of insert thing here.

The change is usually purely biological. Many changes ending with a person wearing ill fitting clothes. In some cases there are mental changes as well, typically this is seen as biologically overwriting a person’s impulse. Pumping someone of estrogen won’t suddenly make them like guys in real life, but it’s an author’s way of making their character fit against their new nature.

Classically this is used by a mad scientist or potion making Witch. The serum could be used by themselves either on purpose or by mistake, or on an unwitting victim.