In Harry Potter and the Order of the Phoenix, at one point Umbridge tries to give Veritaserum to Harry in order to learn the location of Dumbledore and Sirius:
'Very well,' she said, though she looked displeased. 'In that case, you will kindly tell me the whereabouts of Sirius Black.'
Harry's stomach turned over and his hand holding the teacup shook so that it rattled in its saucer. He tilted the cup to his mouth with his lips pressed together, so that some of the hot liquid trickled down on to his robes.
'I don't know,' he said, a little too quickly.
'Mr Potter,' said Umbridge, 'let me remind you that it was I who almost caught the criminal Black in the Gryffindor fire in October. I know perfectly well it was you he was meeting and if I had had any proof neither of you would be at large today, I promise you. I repeat, Mr Potter . . . where is Sirius Black?'
'No idea,' said Harry loudly. 'Haven't got a clue.'
They stared at each other so long that Harry felt his eyes watering. Then Umbridge stood up.
Harry looks really concerned that he might give out Sirius, but this will actually not be possible. 12 Grimmauld place is under the protection of the Fidelius charm and Harry is not the secret keeper. So he should not be able to tell her even if he did drink Veritaserum.