I have seen several threads get started talking about acid reflux and stomach pain with vaping, but the threads just seem to die off without anyone getting to the bottom of it. After a lot of trial and error, I have found that anything containing tart flavoring, especially fruit flavors, will trigger extremely severe burning acid pain in my stomach. It tends to happen after several hours of vaping the same flavor, and once it gets bad, it is extremely difficult to relieve the pain - I have to guzzle lots of Pepto just to take the edge off. I've written to several vendors about this, and even though they are all extremely helpful and concerned, it is difficult to pinpoint the problem, as even the best vendors use flavorings that already have a number of ingredients. I suspect that the culprit may be malic acid, which occurs naturally in many fruits, and is used as a tartness additive in many flavorings. Of course, it does not seem to have this effect when consumed in foods, so perhaps vaping allows it to come into contact and accumulate in areas of the digestive system that it does not affect when consumed. Anyway, I would be very interested in hearing from others who might have this issue.