Sarsaparilla or Smilax Regelii/Smilax Officinalis

Sarsaparilla oil extract is used in modern times to make a popular carbonated sweet beverage. (its Called SARs in Australia). It’s said to taste similar to some cough medicines, maybe because it was used to flavour medicine over the centuries. The most popular species for commercial use is the Jamaican Smilax Regelii. Home made root beer made from sarsaparilla roots is generally more "birch-y" tasting than the commercial brands which add additives like wintergreen oil extract to rid its bitterness.

Many chemicals exist in the root such as oils, smilasaponin, smilax saponins A-C, smiglaside A-E, smitilbin, stigmasterol, acetyl-parigenin, astilbin, beta-sitosterol, caffeoyl-shikimic acids, dihydroquercetin, diosgenin, engeletin, essential oils, epsilon-sitosterol, sarasaponin, sarsaparilloside, eucryphin, eurryphin, ferulic acid, glucopyranosides, isoastilbin, isoengetitin, kaempferol, parigenin, parillin, pollinastanol, resveratrol, rhamnose, saponin, sarsaponin, sarsasapogenin, shikimic acid, sitosterol-d-glucoside, smilagenin, taxifolin, and titogenin. 

Warnings
Large doses may cause gastrointestinal upsets and interact with other medicines.

Medical Uses
Psoriasis lesions, Syphilis, Rheumatism, Wounds, Acne, and other Skin conditions and anti-inflammatory: Drink a decoction as directed in general recipe below, but you can also apply to the skin and effected areas, as a tincture ointment made by soaking the chopped fresh or dried root for days in an alcohol solution and straining into a jar. 
You can buy capsules of the dried ground root, as well from herbalists, as an easier to source. 

General Medical Recipe: ½ to 1 cup of dried roots are boiled in 2-3 cups of stilled water for 10 minutes. This is a decoction opposed to an infusion tea as it needs heat to release the chemicals  although an infusion is classed better because heat will weaken its effectiveness. Hence why never boil longer then needed.