For those too lazy to follow the link:

  "Psilocybin is not considered to be addictive nor does it cause compulsive use. One reason is that the intense experience, which can be physically and mentally challenging, may cause people using psilocybin to limit their frequency of use."

  "Another reason is that the human body quickly builds tolerance to psilocybin, such that people require much higher doses after only a few days of repeated use, making it extremely difficult to have any effect after more than four days of repeated usage."