Overall, there is no clear research about the harm of drinking carbonated water. The difficulty can come with excessive drinking of carbonated cola/other drinks as the sugar in the sweetened varieties is clearly of concern. If a person has irritable bowel syndrome, the added carbon dioxide might cause pain due to bloating. It is true that carbonated water is slightly more acidic than still water due to carbonic acid but this acid decomposes to carbon dioxide and water and is much less acidic than stomach acid. I drink inexpensive soda water every day and aside from the inevitable burp, I have had zero negative symptoms or other untoward reactions to it -- there is no serious literature documentation to the use of carbonated WATER so drink up!