The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
Because it is a herb specifically indicated to treat syphilis as can be seen on Smilax Glabra Root's page.
Its main actions are: Expels Damp-Heat especially from the skin. Clears Heat and dispels Dampness. Dispels toxicity such as Syphilis.