"Three drops of this and even You-Know-Who himself would spill out his innermost secrets. The use of it on a student is -- regrettably -- forbidden."
-- Severus Snape (GF27)
Veritaserum, the most powerful Truth Serum available in the Wizarding world, is a colorless, odorless potion (which looks like plain water) which forces the drinker to tell the truth (HBP9).
References from the canon
- A Truth Potion so powerful that three drops would have the drinker spilling his or her innermost secrets (GF27, OP32). It is both colorless and odorless (HBP9), which is why Professor Umbridge knew she could mix it into Harry's tea (OP28).
- Snape threatened to put some in Harry's evening pumpkin juice to get the truth when he thought the Gryffindors were stealing Polyjuice Potion ingredients as they had done two years before. This time, it was Fake-Moody, getting ingredients to keep his identity a secret (GF27).
- Use of this potion is controlled by very strict Ministry guidelines (GF27). Sirius Black was never offered a chance to take Veritaserum to prove his innocence over the deaths of the Potters and Muggles actually killed by Peter Pettigrew, and Barty Crouch Sr. was so zealous he might not have believed the truth (JKR).
- Dumbledore had Snape use it on Barty Crouch Jr. (GF35). The potion worked because Crouch was overwhelmed and unable to block the effects with Occlumency, sealing his own throat, or transfiguring the potion into something else (JKR). "Veritaserum works best upon the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it" (JKR).
- According to Snape, Veritaserum takes a full moon-cycle to mature when it is being made (OP32). The trustworthiness of this statement may be questioned, since Snape was speaking to Umbridge at the time and may have wished to avoid giving her the potion, but Snape's subtlety is such that even under those circumstances he may have been speaking the truth.
- Dumbledore said that Snape gave Umbridge fake Veritaserum (OP37)
- Discussed during the first class of sixth-year Potions; students who have achieved an N.E.W.T. in Potions should be able to make Veritaserum (HBP9).
- An alert wizard who's good at Potions can protect himself against the use of Veritaserum by keeping some antidote handy (HBP17)
- Rita Skeeter used Veritaserum on Bathilda Bagshot during an interview for the "Life and Lies of Albus Dumbledore" (DH18).
"veritas" L. truth
There are several Muggle versions of Veritaserum which have been used to varying effects to get at the truth, although the ethics are a little sketchy.
Alcohol is the age-old truth serum, as both Muggles and wizards say more than they should when tipsy, as both Hagrid and Mundungus Fletcher surely know. When Harry wanted to get Slughorn's true memory about young Tom Riddle and the Horcruxes, he waited till the professor had imbibed several rounds of drink before he tried to persuade him, and it worked probably better than Veritaserum.
Sodium Pentathol is perhaps the best known Muggle truth-serum, made famous by countless spy stories and movies. It is a barbituate drug that works to overcome the will of a subject and make them more apt to spill their secrets. However, too much of it can kill, and what people say under the influence is not always admissible in a court of law. However, it is probably still used by spy agencies in many countries.
Finally, there is the lie-detector test or polygraph. This device uses sensors placed on the body to measure responses such as heart-rate to determine if lies are being told. The results are also not admissible in courts because some results are inconclusive, and some people can actually control themselves enough to fool the machine (the Muggle version of Occlumency). But it remains a useful tool as police use the polygraph to rule out innocent suspects, because when someone tells the truth they can usually pass easily. (Maybe Harry should have tried that with Snape more often.)
When asked why Veritaserum isn't used in interrogations, Rowling replied:
Q - Why isn’t Veritaserum used in interrogations?
A - It is, but skilled wizards can avoid its effects by using antidotes and charms. A gifted Occlumens could also resist Veritaserum (JKR).