A shrinking potion that causes the drinker to reduce in size and age. Bright green in colour, if brewed incorrectly the potion can be poisonous. Professor Snape threatened to give Neville's incorrectly brewed potion to his pet toad. Luckily Hermione was able to help Neville fix the potion, avoid Trevor's death and the toad turned into a tadpole to everyone's relief.
