Diabetes mellitus is defined by hyperglycemia resulting from impaired insulin action or distress, or both. Metformin is widely prescribed to control the disease and the most common adverse events caused by its use are diarrhea, nausea and/or abdominal discomfort. We evaluated the frequency profile of gastrointestinal adverse events associated with the use of metformin. This is a cross-sectional observational study, in which the occurrence of diarrhea in patients with type 2 diabetes mellitus who started treatment with metformin hydrochloride at a dose of 500 mg of prolonged release was reported. 12 (41.37%) participants were male and 17 (58.62%) were female. Diarrhea was present (n=16) in more than half of the patients (55.2%) who used the drug. Patients had an average of 12 episodes of diarrhea per month. There was also the presence of abdominal discomfort (n=7), in which 24.1% reported being very high. The study showed that the extended-release drug metformin hydrochloride 500 mg can cause gastrointestinal adverse events at the start of treatment and lasts for approximately twelve days.