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.

Inclusion Criteria:

• Male or female outpatients aged 18 to 75 years inclusive
• Symptomatic ulcerative or erosive esophagitis
• Presence of heartburn (daytime and/or nighttime).
• Understanding the study and agreeing to give a written informed consent
• Able to communicate well with the investigator him(her)self or his/her representatives
• Able and agreeing to comply with all study requirements

Exclusion Criteria:

• gastrointestinal bleeding
• gastric or esophageal surgery
• Zollinger-Ellison syndrome
• primary esophageal motility disorders,
• esophageal stricture,
• inflammatory bowel disease,
• upper gastrointestinal malignancy,
• pancreatitis,
• malabsorption
• Barrett's esophagus (> 3 cm)
• Severe disease/condition such as malignancy
• Hypersensitivity to PPIs