It is established that children, especially infants, are subject to adverse drug reactions in view of the physiological immaturity of their renal, hepatic and immune functions as well as the lack of marketing authorization for certain pediatric forms, particularly in neonatology, increases this risk. Currently, for some diseases, children are not treated according to age. Doctors are often required to prescribe medicines designed for adults, which can have serious or even dramatic consequences. Pharmacovigilance aims to ensure the safe use of medicines and to monitor adverse drug reactions not only under normal conditions of use but also in case of medication errors. Here we report four cases of overdose by vitamin D in a prospective study in relation with drugs side effects who was conducted for 6 months in 2013 at children’s hospital of Rabat in Morocco (HER, Hôpital d’Enfants de Rabat). The description of the clinical and paraclinic effects of the four cases was severe dehydration with hypercalcemia and nephrocalcinosis, vomiting, refusal of breastfeeding, weight loss. The four infants had taken a dose of 600,000 international units (UI) during the first fifteen days of life. They were hospitalized, followed a corrective treatment, the evolution after treatment was favorable with a follow-up of growth.