Horse chestnut (Aesculus hippocastanum) is a tree. Horse chestnut contains significant amounts of a poison called esculin and can cause death if eaten raw.
Horse chestnut also contains a substance that thins the blood. It makes it harder for fluid to leak out of veins and capillaries, which can help prevent water retention (edema). The horse chestnut fruits contain seeds that look like the sweet chestnut but have a bitter taste.
People most commonly take horse chestnut seed extracts by mouth to treat poor circulation that can cause the legs to swell (chronic venous insufficiency or CVI). It's also used for many other conditions, but there is no good scientific evidence to support these other uses.
