This is what I read on the internet and it may have been on drugs.com regarding Guaifenesin : "increase gastric acidity and may also relax lower esophageal sphincter, which can lead to gastric reflux into the esophagus."
I didn't want to take a chance of causing me to make things worse in terms of acid etc. since I have the hiatal hernia and then the possibility of relaxing the lower sphincter..... if in fact my lower sphincter is working. As far as I know, with the gastroenterologist tests, it is working, but...for me... I don't want to chance it. So hard to know sometimes what is best to do.