Knotweed, commonly referring to species such as Polygonum cuspidatum (Japanese knotweed), has a history of use in traditional East Asian herbal medicine. Traditional practices, particularly in Chinese and Japanese systems, have employed knotweed for its purported cardiovascular and circulatory benefits. Its use is often attributed to the presence of resveratrol and other polyphenolic compounds, which are believed to have vasoprotective and antioxidant properties. Historical texts describe knotweed preparations being used for conditions involving "blood stasis" and to support healthy blood flow, which can be interpreted as attempts to support capillary and vascular health.

However, while there is some modern scientific interest in resveratrol for endothelial and vascular function, direct studies on knotweed extracts and their effects on human capillaries are limited and mostly preclinical. The traditional use, therefore, remains the primary basis for its application in supporting the capillaries body system. Scientific validation is still emerging, and current evidence from controlled human studies is lacking or indirect. As such, the evidence rating is moderate, reflecting traditional but not robust scientific support.