Metformin is one of the most used antidiabetic drugs worldwide and is not often expected to develop serious side effects. Our study presents a case of abnormal uterine bleeding in a 19-year-old female patient who developed it after starting metformin. The patient was evaluated further, and no uterine or haematological pathology was detected. The Naranjo adverse drug reactions scale was used, and the patient was included in the possible group by scoring 7 out of 10. It should be remembered that metformin may cause bleeding in some patients due to its effects on the coagulation system.