Horse chestnut (Aesculus hippocastanum) seed extract has a documented history of use for supporting vascular health, particularly in the capillaries and veins. Its primary active component, aescin (or escin), has been studied for its effects on chronic venous insufficiency (CVI), a condition characterized by poor blood flow and capillary leakage in the legs. Several randomized controlled trials and meta-analyses have shown that horse chestnut seed extract can significantly improve symptoms associated with CVI, such as leg swelling, pain, and heaviness, when compared to placebo. The extract is believed to help strengthen capillary walls, reduce capillary permeability, and promote venous tone, which collectively supports capillary function and reduces edema.

A 2012 Cochrane Review concluded that horse chestnut seed extract is more effective than placebo for treating symptoms of CVI, with effects comparable to compression stockings in some studies. The mechanism is thought to involve aescin inhibiting enzymes that break down capillary walls and reducing inflammation. There is less evidence for its use in other capillary-related disorders, but for venous insufficiency and related capillary dysfunction, the scientific support is robust. However, horse chestnut should be used with caution, as raw seeds and some preparations can be toxic if not properly processed.