Veritaserum is an incredibly strong potion, which forces the drinker to be truthful while the potion is in effect. It works best when the person on whom it is used it in a weak or vulnerable state.
 Appearances in the Books
 Fourth Book
Severus Snape first introduces the potion to Harry Potter when threatening to use it on him after someone has been stealing from his store cupboard. In truth this was Barty Crouch Jr., however Snape suspected Harry, as two years previously he had provided a distraction so that Hermione Granger could do just that. Though again Snape had suspected Harry. Dobby having recently stolen Gillyweed for Harry didn't help either.
Albus Dumbledore Stunned and then used Snape's Veritaserum on Barty Crouch Jr., a Death Eater who had been impersonating Alastor Moody at Hogwarts. Under its influence Crouch told his story and confessed.
 Fifth Book
Dolores Umbridge asks for Veritaserum from Severus Snape to interrogate Harry Potter. Snape provides fake Veritaserum on behalf of the Order of the Phoenix, however Harry suspected Umbridge and only pretended to drink it. Umbridge used an entire bottle on Harry, ignoring Snape's advice on how to use it.
 Sixth Book
 Other Uses
 Outward Appearance
Veritaserum is a colourless liquid, with a water-like consistencey.
 Additional Canon
Veritaserum works best upon the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it. Barty Crouch had been attacked before the potion was given to him and was still very groggy, otherwise he could have employed a range of measures against the Potion - he might have sealed his own throat and faked a declaration of innocence, transformed the Potion into something else before it touched his lips, or employed Occlumency against its effects. In other words, just like every other kind of magic within the books, Veritaserum is not infallible. As some wizards can prevent themselves being affected, and others cannot, it is an unfair and unreliable tool to use at a trial.
Sirius might have volunteered to take the potion had he been given the chance, but he was never offered it. Mr. Crouch senior, power mad and increasingly unjust in the way he was treating suspects, threw him into Azkaban on the (admittedly rather convincing) testimony of many eyewitnesses. The sad fact is that even if Sirius had told the truth under the influence of the Potion, Mr. Crouch could still have insisted that he was using trickery to render himself immune to it.
- ↑ 1.0 1.1 J. K. Rowling's Official Website, Section: F.A.Q., Veritaserum plays a big part in finding out the truth from Mad-Eye Moody in book four. Why then is it not used for example in the trials mentioned in the same book? It would be much easier in solving problems like whether Sirius Black was guilty or not?
- ↑ 2.0 2.1 J. K. Rowling, GoF 27
- ↑ J. K. Rowling, CoS 11
- ↑ J. K. Rowling, GoF 26
- ↑ J. K. Rowling, GoF 35
- ↑ J. K. Rowling, OotP 28
- ↑ 7.0 7.1 J. K. Rowling, OotP 32
- ↑ J. K. Rowling, HBP 9