Birch oil or extract, derived from the bark of sweet birch trees (Betula lenta), often called black or cherry birch, is primarily known for its unique flavor profile that is both sweet and minty but reminds most people of root beer. In the beverage industry, its most notable application is in the production of birch beer, which was traditionally made by brewing and fermenting birch sap or by using birch bark extract. Today, it most likely produced using methy salyicate (wintergreen).
It should be noted that birch oil is 99% methyl salyicate, which is closely related to Aspirin, and can be toxic and potentially fatal to children if not properly formulated.