Polygala, particularly Polygala tenuifolia (known as Yuan Zhi in Traditional Chinese Medicine), has a long history of use in East Asian herbal medicine for respiratory conditions, including dry cough. Traditional texts describe its ability to "transform phlegm," calm the mind, and relieve coughs that are associated with dryness or irritation of the lungs. The roots of Polygala are believed to moisten the lungs and have expectorant properties, making them popular in formulations aimed at alleviating persistent, dry coughs.

Scientific research on Polygala’s efficacy for dry cough is limited, but some pharmacological studies have identified saponins and other compounds that may possess expectorant and antitussive (cough-suppressing) activities. Animal studies have shown that extracts from Polygala species can increase respiratory tract secretions and reduce coughing frequency, supporting some traditional claims. However, these findings are preliminary, and robust clinical trials in humans are lacking. Thus, while there is some biological plausibility, the justification for its use remains primarily traditional, with moderate supporting evidence.

In summary, the use of Polygala for dry cough is principally grounded in traditional herbal medicine, supported by some preclinical evidence but lacking strong scientific validation in the form of high-quality human studies.