I Have Continuous, Acid Secretion In My Mouth, Sometimes Foam,

I have continuous, acid secretion in my mouth, sometimes foam, since Nov. 2014. Had upper gastrointestinal endoscope, upper gastrointestinal series, and esophageal test which ruled out acid reflux. GI dr. concluded this was coming from my mouth due to medication, especially narcotics. I have MS, in Constant severe pain, I take and have taken lots of medications, have tried every narcotic available, unfortunately none have worked. Was told there wasn't anything he could do. I do need some relief from this constant burning in mouth and in chest. I know this acid is damaging my esophagus and keepIng it raw. Did try GERD medication and the swish and swallow, trying to eat alkaline food and not eat acid producing foods.