The incident happened on Sunday at an apartment complex, where police say he got the drugs from a friend staying in the home.
The 24-month-old boy was rushed to the hospital after his parents called 9-1-1.
"It was a friend who was staying in the house who was sleeping on the couch," Officer Joe Ramirrez told CBS Denver affiliate KCNC-TV. "He had the mushrooms in a backpack that was accessible to the minor, and it appears that that is the way that the minor got into the mushrooms, by taking it from the backpack."
The parents have not been charged and investigators are on the lookout for that friend.
The Denver Police's child abuse unit is investigating the incident.