Inclusion Criteria:

1. Patients are aged greater than 20 years who have H. pylori infection without prior
eradication therapy and are willing to receive the sequential therapy will be eligible for
enrollment.

Exclusion Criteria:

1. children and teenagers aged less than 20 years,

2. history of gastrectomy,

3. gastric malignancy, including adenocarcinoma and lymphoma,

4. previous allergic reaction to antibiotics (amoxicillin, clarithromycin, levofloxacin,
metronidazole) and prompt pump inhibitors (lansoprazole),

5. contraindication to treatment drugs,

6. pregnant or lactating women,

7. severe concurrent disease,

8. Patients who cannot give informed consent by himself or herself.