Tehran University of Medical Sciences Iranian Journal of Psychiatry 1735-4587 17 4 2022 09 14 476 479 Veysel Özdağ Istanbul University, Istanbul Medical Faculty, Child and Adolescent Psychiatry Department, İstanbul, Turkiye. Oğuzhan Koyuncu Istanbul Medeniyet University, Faculty of Medicine, Child and Adolescent Psychiatry Department, Istanbul, Turkiye Yahya Esad Özdemir Istanbul University, Istanbul Medical Faculty, Child and Adolescent Psychiatry Department, İstanbul, Turkiye. Yaşar Tanir Istanbul University, Istanbul Medical Faculty, Child and Adolescent Psychiatry Department, İstanbul, Turkiye. 2021 08 13 Objective: Methylphenidate, a psychostimulant agent, is used in first-line psychopharmacological treatment in children and adolescents with attention-deficit/hyperactivity disorder. Common side effects associated with methylphenidate use in children and adolescents are insomnia, anorexia, headache, and nausea. Thrombocytopenia, nasal bleeding and menstrual bleeding disorders are very rarely reported during methylphenidate use. One of the least expected side effects during methylphenidate usage is menorrhagia. Method: In this article, we report methylphenidate monotherapy-induced menorrhagia in two adolescent identical twins. To our knowledge, this is the first report of menorrhagia associated with methylphenidate use in children and adolescents. Results: In both cases, menorrhagia has started after methylphenidate monotherapy and stopped after discontinuation. Other possible etiologies have excluded with clinical and laboratory evaluations. Naranjo Adverse Drug Reaction Probability Score was found 7, indicates probable side effect. Conclusion: Menorrhagia is a rare adverse effect of methylphenidate use and clinicians should be aware of this phenomenon. https://ijps.tums.ac.ir/index.php/ijps/article/view/2859 https://ijps.tums.ac.ir/index.php/ijps/article/download/2859/1082