Euphorbias contain a toxin that is known to be carcinogenic. The milky sap can cause inflammation of the skin and mucous membranes of the eyes, nose and mouth. The chemical resinferatoxin has an irritating effect which is stronger than capsaicin, the substance that makes chillies hot. So never feed to tortoises and wear protective clothing when handling this plant.
Candelabra Cactus (Euphorbia lactea)
Milk bush (Indian Tree Spurge) (Euphorbia tirucalli)
N.B. Euphorbia canariensis, which is toxic, looks very like many species of cactus that grow in column shapes and which are safe for tortoises (for example, Cereus, Pachycereus, Hylocereus, Neobuxbaumia, Browningia, and many others). Unless you can accurately identify the species of any plant do not feed it to your tortoise.