The eradication rate of the standard H. pylori eradication therapy (such as the triple therapy with a proton pump inhibitor [PPI], amoxicillin and clarithromycin) depends on bacterial susceptibility to clarithromycin and genotypes of CYP2C19 in patients. The investigators intend to investigate whether the tailored therapy based on the two above-mentioned factors increases the cure rate of the initial eradication therapy.