The cacti contains a vide array of alkaloids but the main active principle is mescaline. Mescaline is a phenetylamine and thus closer related to extacy than the "classic" tryptamine derived halucinogens such as psilocin and DMT.
The dosage for the fresh plant is betweenfour and fifteen "buttons"(the plant is called button due to its form and small size). the Mescaline content of this plant varies greatly(0,5-6%) but is probably most often in the range of 1-2%