A small non-magical worm that sucks blood.
- Leeches are used in Polyjuice Potion, apparently whole (Pm).
- Leech juice is used in Shrinking Solution .
The author wrote on Pottermore that she chose this ingredient for Polyjuice Potion because leeches "suck the essence out of one and into another" (Pm).
MIddle English for "blood-sucking worm"
Leeches have been used medicinally for thousands of years, going back to ancient India and Greece. The leech was attached to a patient and allowed to suck their blood, which was thought beneficial for removing "bad humors" that caused the illness.