Baphicacanthus cusia (also known as Strobilanthes cusia or "Qing Dai" in traditional Chinese medicine) has a documented history of use in traditional medicine for treating various viral infections, including herpes. Recent scientific studies have provided some validation for its antiviral properties. Several in vitro experiments have shown that extracts from Baphicacanthus cusia possess inhibitory effects against herpes simplex virus (HSV), particularly HSV-1. Key studies have demonstrated that indigo naturalis (the powdered extract of the plant) and its active constituents, such as indirubin and tryptanthrin, can suppress HSV replication and reduce cytopathic effects in cultured cells.

For example, research published in peer-reviewed journals between 2010 and 2020 has shown that water and ethanol extracts of Baphicacanthus cusia inhibit HSV-1 replication in vitro. The proposed mechanisms include interference with viral attachment and entry, as well as modulation of host immune responses. However, most of the evidence is preclinical, with limited or no high-quality human clinical trials available to date.

In summary, the use of Baphicacanthus cusia for herpes is supported by both traditional use and scientific studies, but the current scientific evidence is mostly limited to laboratory studies. The evidence rating is moderate (3/5) due to the lack of robust human clinical trials.