STU-Na belongs to the proton pump inhibitors (PPI), a group of drugs that reduce gastric acidity. PPI are used to treat acid related diseases like erosive or ulcerative esophagitis. This trial aims to find out the therapeutic dose of STU-Na required for healing patients with erosive or ulcerative esophagitis. One of four dosages of STU-Na (15 mg, 30 mg, 60 mg, or 90 mg daily), or Esomeprazole 40 mg daily, an already marketed PPI, will be given to patients. The attribution to one of the 5 treatment groups will be by chance. Neither the patient nor the study physician will know, which treatment is administered to the patient.