To the Editor:We read the study on the efficacy and safety of tenapanor in Japan with interest. We present a few concerns.Our first concern is about the reason for the withdrawal of patients from the study in the 60 mg per day titration group. In the discussion, the stated reason for these withdrawals was diarrhea. In the 60 mg per day dose-titration group, the dose was adjusted according to the gastrointestinal symptoms. Therefore, we think that a factor other than diarrhea may have caused the withdrawals in the 60 mg per day dose-titration group. We believe that further clarification and investigation into the reason for these withdrawals could help to reduce the number of withdrawals in clinical practice.Our second concern is about the appropriateness of the minimum dose. In a previous study conducted in the United States, the efficacy was confirmed in the 6 mg per day group. This would suggest that perhaps a lower dose than the minimum dose used in this study would have been a better choice.Finally, our third concern is about a typographical error. In Table 1, the number of people taking vitamin D in the placebo group ought to be 33, and not 3.