Horse chestnut (Aesculus hippocastanum) seed extract is used to support or treat capillary weakness, particularly in the context of chronic venous insufficiency (CVI), varicose veins, and associated symptoms such as edema, swelling, and leg heaviness. The primary active constituent identified is aescin (also called escin), a mixture of saponins. Scientific studies suggest that aescin exerts a venotonic effect, improving the tone of veins and reducing capillary permeability, thus decreasing fluid leakage and swelling.

Multiple randomized controlled trials and meta-analyses have examined the efficacy of horse chestnut seed extract for venous disorders. A 2012 Cochrane review concluded that horse chestnut extract is more effective than placebo in reducing leg pain and edema in CVI, with a safety profile comparable to placebo. Mechanistically, aescin appears to strengthen capillary walls by inhibiting the activity of enzymes (such as hyaluronidase and elastase) that break down capillary wall components, and by reducing inflammation and improving microcirculation.

Although most research focuses on venous insufficiency, the underlying rationale—improving capillary strength and reducing permeability—also applies to general capillary weakness. Therefore, there is moderate to strong scientific support (rated 4/5) for the use of horse chestnut in supporting capillary health, especially in conditions where capillary fragility and leakage contribute to symptoms.