Hmmm they aren't poisonous but the are psychoactive. They contain psilocybin, psilocin and serotonin. In very small amounts they would produce a feeling of euphoria and heightened awareness, colours are also likely going to seem brighter. In larger amounts they would produce hallucination's and vivid dream like states of conciouness.
A small amount would be like half or maybe a whole cap, I guess a large amount would be a whole mushroom.