 A. phalloides is one of the most poisonous of all known toadstools . It is estimated that as little as half a mushroom contains enough toxin to kill an adult human . It has been involved in the majority of human deaths from mushroom poisoning , possibly including the deaths of Roman Emperor Claudius in AD 54 and Holy Roman Emperor Charles VI in 1740 . It has been the subject of much research , and many of its biologically active agents have been isolated . The principal toxic constituent is α @-@ amanitin , which damages the liver and kidneys , causing hepatic and renal failure which can be fatal . 
