Some treatments for sour or upset stomach include certain foods, such as bananas, rice, applesauce and toast, and supplements, such as calcium and magnesium. The latter two are particularly effective, according to WebMD. A non-oral treatment is to place a heating pad or hot water bottle on the stomach, although this should be used in moderation to avoid damaging the skin.
Apple cider vinegar is another potential treatment for a sour or upset stomach, according to Healthline. It may also be used as a preventative measure when taken in a dose of a spoonful per day. Peppermint is another treatment, which works as a natural analgesic.