Reducio' is the counter-spell for Engorgio. It causes an Engorged object to return to its normal size.
- In 1994 when Hermione Granger's teeth were enlarged by the Densaugeo jinx accidentally by Draco Malfoy, Madam Pomfrey probably used Reducio to return her teeth back to normal size, though Hermione took advantage of this situation and allowed her teeth to be shrunk to less than their original size, up until this point she was always said to have "rather large front teeth"
- In 1994 Barty Crouch Jr. (disguised as Alastor Moody), who had enlarged a spider with an Engorgement Charm, used this to return it to its normal size.
- In 1998 Harry Potter was having trouble with this charm along with the Engorgement Charm when casting them upon a spider in order to practise with the Blackthorn wand Ron had won from a group of Snatchers, his attempts were feeble as he was not the real master of the wand.
- Harry Potter may have used this spell unintentionally to shrink an ugly sweater of Dudley's when Aunt Petunia was trying to force it over his head.
 Possible Etymology
English reduce, "to shrink". (Latin has a verb reducere, present tense reduco. This is the source of the English "reduce", but has a different meaning.) Also in Italian Riduco first person present tense of Ridurre, same root of Latin Reducere.
 Behind the scenes
- In Harry Potter and the Deathly Hallows: Part 1, the scene where the spider is enlarged is replaced by one of Hermione's bluebell flames.