Muggles' Guide to Harry Potter/Magic/Veritaserum

Overview
Veritaserum is a colorless, odorless potion that forces the drinker to tell the truth.

Analysis
Since the wizards have access to an infallible truth serum, some readers have questioned why this cannot be used by the Wizengamot to determine whether a suspect is innocent or not; and it has also been suggested that it could have been used in Harry Potter and the Half-Blood Prince in order to force Professor Slughorn to divulge what he knew about the young Tom Riddle. However, anything so powerful would be bound to cause creation of counter-potions and counter-charms, just as a means for editing memories has sprung up in the wake of the creation of a device (the Pensieve) for playing them back; and the author has stated, firstly, that Slughorn, being so powerful and cautious a wizard, is bound to have secreted about himself potions to counteract the effects of Veritaserum and other harmful (to him) potions; and second, that the existence of charms that prevent or alter the action of Veritaserum on suspects would make Veritaserum-induced evidence invalid in the Wizengamot.

It is certainly true that, while Veritaserum is mentioned several times in the books, it is only successfully administered once, to an unprepared, Stunned, and effectively unsuspecting victim, in Harry Potter and the Goblet of Fire, when Professor Dumbledore used it on Barty Crouch Jr.