Gastrointestinal side effects of proton pump inhibitors (PPI) medicines can be diarrhea and constipation. The purposes of this research were to evaluate gastrointestinal side effects namely diarrhea and constipation in inpatients at Gatot Soebroto Hospital. This descriptive analytic study collected data prospectively from prescriptions and patient medical records and through an interview with valid and reliable questionnaire. The data were collected from February to April 2016 by total sampling method. Causality analysis of gastrointestinal side effects was done by using Naranjo Algorithm while Chi-square and Fisher absolute test were used to see the association between gastrointestinal side effect and gender, age, PPI’s dose, and duration of administration of PPI. Samples were patients who were ≥ 17 years old, received proton pump inhibitor and signed the Informed Consent. From 58 patients as research samples, nineteen patients (32.75%) experienced constipation which 16 patients (27.58) and 3 patients (5.17%) were with probable and possible category, respectively. None experienced diarrhea. There were no significant association between constipation and sex, age, PPI’s dose and the duration of PPI’s.