Conkers (horse chestnuts) are poisonous to humans if eaten raw. They contain a toxic substance called the glucoside aesculin, which can cause gastrointestinal problems. While not usually fatal, eating conkers can make a person ill with symptoms such as nausea, vomiting, and abdominal pain. The toxicity level is high enough that they are considered unfit for human consumption, and even small amounts can cause adverse effects. Cooking and specific processing were historically used to make them safer for consumption, but this is uncommon today. Conkers should not be eaten by humans or given to most animals, as they can cause serious health issues.
Severe toxic reactions, including vomiting, dizziness, allergic reactions, and even heart-related symptoms, have been reported in some cases of horse chestnut seed ingestion.