Toronto Public Health says the patient consumed a type of wild mushroom identified as Amanita, which it says can be fatal.
An official refused to comment on the patient's condition, citing confidentiality.
In a statement, the agency reminds the public not to pick and eat mushrooms found growing in the wild. It says some varieties may look similar to mushrooms that are safe to consume, but are in fact poisonous.
Symptoms most commonly associated with eating poisonous mushrooms include nausea, vomiting, stomach cramps and diarrhea. More severe symptoms include sweating, convulsions, hallucinations and coma.
Health officials say deaths are rare and most people recover from eating poisonous mushrooms, but residents should play it safe.
"While it may be tempting for some residents to forage for wild mushrooms, it can be difficult to determine if mushrooms growing in the wild are edible or poisonous," said Dr. Margaret Thompson, medical director of the Ontario Poison Centre at the Hospital for Sick Children in a statement.
"Our advice is to not take the risk."