Birch beer in its most common form is a carbonated soft drink made from herbal extracts, usually from birch bark. It has a taste similar to root beer (Wikipedia, nd).
Although uncommon, an allergic reaction to birch beer may still occur, due to the use of herbal flavors. In the event of an allergic reaction after consuming birch beer, contact your allergist.
Birch beer is made by dozens of brands, and can usually be found in specialty food stores. However, it is considered an uncommon soft drink selection, so it may not appear at major retail locations.
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Here is a great list of the top birch beers you can buy!