Horse chestnut (Aesculus hippocastanum) seed extract is primarily used for chronic venous insufficiency (CVI) and related symptoms such as leg swelling and varicose veins. The active compound, aescin (or escin), has demonstrated anti-inflammatory, antioxidant, and vasoprotective effects. There is some scientific evidence suggesting that aescin may help reduce blood viscosity and improve venous tone, which in turn could theoretically help prevent blood clots (thrombosis), especially in the context of venous stasis and chronic vascular insufficiency. Small clinical studies and systematic reviews have found horse chestnut seed extract effective for CVI, reducing leg swelling and discomfort, but direct evidence for its role in the prevention of blood clots is limited and indirect.

The mechanism by which horse chestnut may influence clot formation includes improvement in venous return and mild anti-edematous effects. Some in vitro and animal studies suggest escin can reduce platelet aggregation and improve microcirculation, but robust clinical trials specifically targeting blood clot prevention in humans are lacking.

Therefore, while horse chestnut is sometimes suggested to support vascular health and theoretically could help reduce clot risk in people with CVI, it should not be relied upon for the prevention or treatment of blood clots. Its use is better supported for symptoms of venous insufficiency rather than as an antithrombotic agent. Always consult a healthcare provider before using it for this purpose.