"It is Veritaserum - a Truth Potion so powerful that three drops would have you spilling your innermost secrets for this entire class to hear," said Snape viciously. "Now, the use of this potion is controlled by very strict Ministry guidelines. But unless you watch your step, you might just find that my hand slips" - he shook the crystal bottle slightly - "right over your evening pumpkin juice." (The Goblet of Fire, 517)
It's also only as accurate as the person's mind who has drank the potion. Therefore, if a person really believes something, they will be able to easily say it, even if it isn't true. Veritaserum is used in The Goblet of Fire on Barty Crouch Jr., who is not exactly in a clear state of mind:
"As Minerva and Severus have doubtless told you," said Dumbledore, "we heard Barty Crouch confess. Under the influence of Veritaserum, he told us how he was smuggled out of Azkaban, and how Voldemort - learning of his continued existence from Bertha Jorkins - went to free him from his father and used him to capture Harry. The plan worked, I tell you. Crouch has helped Voldemort to return."
"See here, Dumbledore," said Fudge, and Harry was astonished to see a slight smile dawning on his face, "you - you can't seriously believe that. You-Know-Who - back? Come now, come now...certainly, Crouch may have believed himself to be acting upon You-Know-Who's orders - but to take the word of a lunatic like that Dumbledore..." (704)
Of course, Fudge would've looked for any loophole possible in denying Voldemort's return (he also says Harry is lying). For the Dark Lord to return while he was Minister of Magic would be horrible for his career, so he refuses to believe Harry, despite Dumbledore's support and explanation of Barty Crouch Jr's confession.
In the end, the Veritaserum helped uncover the truth, but still no one would believe it for quite some time.
Join me tomorrow for W is for the Will of Dumbledore!