Why would depression medication lower or obliterate reflux problems? My friend is reflux free now that she is on depression medication, and the change was immediate. She has had major reflux and excess acid problems for over 15 years and the last year had reflux problems even after drinking water. She's taken most of the reflux medications out there until the doctor pressed her to take depression medicine (the doctor felt she was depressed) my friend believes she was stressed not depressed.
The short answer is it's hard to say for sure. Different types of antidepressants have various direct gastrointestinal side effects. Serotonin reuptake inhibitors (such as fluoxetine, sertraline, citalopram, and paroxetine) may increase gut motility a little bit (some patients have loose bowel movements or diarrhea as a side effect).
In your friend's case, perhaps improved emptying of the stomach is reducing the tendency to reflux. Alternatively, some medicines such as tricyclic antidepressants have anticholinergic and antihistamine-like effects, altering nervous sytem stimulation of the acid-secreting cells of the stomach, and these effects may reduce gastric acid production.
Finally, when people are stressed out they may have increased gastric acid output, and the anti-stress effects of antidepressants may be helpful in reversing this problem.