Tree Peony (Paeonia suffruticosa) root bark, known as "Mu Dan Pi" in Traditional Chinese Medicine (TCM), has a long history of use for treating various inflammatory skin conditions, including rashes and hives (urticaria). In TCM, Tree Peony is believed to "clear heat" and "cool the blood," concepts that often relate to reducing inflammation and calming skin eruptions such as rashes and hives.

Historical TCM texts and modern TCM practice guidelines frequently list Mu Dan Pi among herbal formulations for skin conditions characterized by redness, swelling, and itching. While there is strong traditional support for its use, scientific evidence remains limited. Some preclinical studies have shown that extracts of Tree Peony contain compounds (e.g., paeonol) with anti-inflammatory and immunomodulatory effects, which could theoretically help with allergic reactions and skin inflammation. However, these findings are primarily from laboratory or animal studies, not robust clinical trials.

In summary, Tree Peony’s use for rashes and hives is primarily justified by traditional medicine practices, with some preliminary—but insufficient—scientific support. High-quality human studies confirming its efficacy for these specific skin conditions are lacking.