Psilocybin has an LD 50 of 280mg/kg. In comparison, the LD 50’s of LSD, THC (the active compound in marijuana), and mescaline are 30mg/kg, 42mg/kg, and 370 mg/kg. Thus, when death is considered as the toxic endpoint, psilocybin is one of the least toxic of the hallucinogens.
Shroomery - Psilocybe Toxicity Information
http://www.shroomery.org/6297/Psilocybe-Toxicity-Information