Leech juice

Leech juice is, most likely, liquefied Leeches. It's a purple liquid and it's used in potion-making: in the Shrinking Solution a dash of leech juice is one of the ingredients (adding too much, however, causes the potion to turn orange instead of the bright acid green it's supposed to be; this can prove disastrous, as making errors in the potion can cause the mixture to become poisonous).

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (video game)