Horse chestnut may increase the risk of bleeding or potentiate the effects of warfarin therapy.

– Am J Health Syst Pharm 2000 Jul 1;57(13):1221-7; quiz 1228-30 —
Potential interactions between alternative therapies and warfarin. — Heck AM, DeWitt BA, Lukes AL. — School of Pharmacy and Pharmacal Sciences, Purdue University, Indianapolis, IN, USA.

May cause contact dermititis.
– Contact dermatitis to extract of horse chestnut (esculin). — Comaish JS, et al. — Contact Dermatitis. 1980 Jan;6(2):150-1.

May cause an allergic reaction if you have an allergy to pollen.
– Allergy 1992 Aug;47(4 Pt 2):380-3 — Horse chestnut (Aesculus hippocastanum) pollen: a frequent cause of allergic sensitization in urban children. — Popp W, Horak F, Jager S, Reiser K, Wagner C, Zwick H.

May cause gastrointestinal distress or nausea.