Severus Snape
Pumpkin juice is readily available, and can be purchased on the Hogwarts Express. Severus Snape threatened to Harry in his fourth year that he might slip Veritaserum in his morning pumpkin juice while believing that Harry had stolen some of Snape's potion ingredients. Prior to a Quidditch match in his sixth year, Ron Weasley believed that Harry had slipped Felix Felicis into his morning juice to help him play perfectly.