Gastric acid reflux. Artwork of the human stomach depicting gastro-oesophageal reflux. This is caused by poor functioning of the muscles of the lower oesophagus (top centre), which permits reflux (regurgitation) of acid (yellow) from the stomach (centre) into the oesophagus. Gastric acid reflux may cause inflammation of the oesophageal mucosa (oesophagitis). The main symptom is heartburn, which may be worsened by smoking, drinking alcohol or by obesity.