Quassia is a plant. The wood is used as medicine. Quassia is used for treating an eating disorder called anorexia indigestion constipation and fever. It is also used to rid the intestines of various kinds of worms; as a tonic or purgative; and as a mouthwash. Some people apply quassia directly to the scalp for lice. Rectally quassia is used for treating worm infestations.