Traditionally, Sassafras has been used to make root beer! Sassafras is a tree native to North America. These trees can grown over 100 ft tall, and have historically been used for a number of medicinal purposes.
The FDA banned sassafras as a carcinogen in 1960 due to concern about safrole, which is found in the oil of sassafras. Some people believe that ruling is controversial due to a study where rats ate high doses of safrole. Due to the lack of a root beer lobby in the US, we may never know the truth about sassafras for sure!
Because of this, commercial root beer is no longer made with sassafras, but with artificial and/or natural flavors similar to sassafras. Sweet birch and sarsparilla are similar plants with similar flavors that are often used. Birch is also the primary flavor in birch beer, and sarsparilla is the primary flavor in sarsparilla, which are both very similar to root beer. One big difference is that root beer typically has a blend of other interesting flavors making a diverse flavor profile, like: vanilla, cinnamon, cloves, anise, nutmeg, wintergreen, honey, molasses, and more!