Initial Posting: 2/10/2017    Last Update: 7/25/2017
Answer: As discussed in our article about Horse Chestnut, stomach irritation can be caused by a compound naturally found in horse chestnut -- even in extracts. By taking the extract in a controlled release formulation, the incidence of irritation may be reduced. So if the product you used was not controlled release, the side effects you experienced may not be unusual.
Horse chestnut is typically used to reduce inflammation and swelling, particularly associated with varicose veins and hemorrhoids. Be aware that consuming "whole" horse chestnut (as opposed to extracts from which a toxin, esculin, has been removed) is dangerous. For more information about uses, evidence, dosing, side effects, and drug interactions, see the Horse Chestnut article.