Veritaserum is a potion that makes the drinker tell the truth. It is sometimes used for taking testimony in judicial investigation. Veritaserum does not only prevent the drinker from lying but compels them to answer questions.
Veritaserum is a very powerful potion. 2 drops is the usual dose for someone who is not an occlumens (canon) and 3 drops for an occlumens, as occlumency allows to resist a smaller dose.
Toxicity[]
Any dose larger than 3 drops of veritaserum is considered toxic. A large dose of veritaserum can cause permanent insanity (or death ?) unless an antidote is administered promptly.
Uncontrollable utterance ailment[]
People who have been poisoned with veritaserum (even if given the antidote) can suffer from uncontrollable utterance ailment: a compulsion to tell random thoughts at any time. This is known as a distressing and debilitating disorder as it can prevent the affected person from sustaining normal chains of thought.
In 1999, Nymphadora Tonks suffers from uncontrollable utterance ailment after Bellatrix Black makes her drink a full draught of veritaserum during her attack on the Tower.
Harry call this "PrakW syndrome" in reference to muggle novel Life, the Universe and EverythingW by Douglas AdamsW.