Spirotriterpenoid and Ingenol mebutate
The genus contains over 2,000 diverse species including poinsettia and some cactus-like species.
Most spurges contain a milky sticky latex which contains a number of different chemical compounds. This is poisonous and acts as a deterrent to herbivores. It oozes from any wound to the plant and congeals on contact with air - effectively gumming up the mouthparts of insects attempting to graze the plant. The latex becomes toxic once exposed to sunlight (phototoxic) and is irritating to the skin and mucous membranes.
The latex was used as a purgative and the name spurge refers to this ("espurge" is Middle English/Old French "to purge").
A compound isolated from petty spurge (Euphorbia peplus), ingenol mebutate, is used to treat actinic keratosis, a skin condition which can develop into a non-melanoma form of skin cancer. The plant is native to Australia where it is commonly known as cancer weed and is used as a traditional remedy for skin cancer.