Varenicline - Does Champix cause heartburn?

Gastrointestinal
Very common (10% or more): Nausea (up to 40%), vomiting (up to 11%)

Common (1% to 10%): Abdominal discomfort, abdominal distension, abdominal pain, abdominal pain upper, constipation, diarrhea, dry mouth, dyspepsia, flatulence, gastroenteritis, gastroesophageal reflux disease, toothache

Uncommon (0.1% to 1%): Aphthous stomatitis, change of bowel habit, dysphagia, eructation, gastritis, gastrointestinal hemorrhage, gingival pain, hematochezia, mouth ulceration

Rare (less than 0.1%): Abnormal feces, acute pancreatitis, enterocolitis, esophagitis, gallbladder disorder, gastric ulcer, hematemesis, intestinal obstruction, tongue coated[Ref]

Source: https://www.drugs.com/sfx/champix-side-effects.html