Phenytoin is an Anticonvulsant mainly acts by promoting Na+ efflux or decreases Na+ influx from membranes in motor cortex neurons; stabilizes the neuronal membrane. Slows conduction velocity.Indicated in Seizures. A child of 12 years old of female patient came to pediatrics department with chief complaints of seizures not associated with fever. Generalised tonic clonic activity with loss of consciousness for 10 min and admitted in pediatrics department-II and his treatment chart was phenytoin 100mg PO OD and valproic acid 200mg – 200 mg – 200 mg Po 2tablets TID. During his second day of treatment child developed fever of 1020F and cough and to reduce the condition physician prescribed paracetamol 500mg Po BD and syrup chlorpheneramine maleate 5ml Po BD and increased the phenytoin dose 100 mg 2 tablets OD. On the 12th day of treatment child was irritable with hyperactive behaviour. Better vigilance is necessary for implementation of safe and effective treatment for each individual patient.in order to prevent serious adverse drug reactions of this drug, close monitoring drug treatment course, creating awareness, recognition of the problem and careful management of all the patients who receive medication are essential, because use of phenytoin causes Drowsiness, Fatigue, Ataxia, Irritability, Headache, Restlessness, Slurred speech, Nervousness, Nystagmus, Dizziness, Vertigo, Dysarthria, Paresthesia, Rash, Pruritus, Gingival hyperplasia (pediatric patients), Ataxia, Paradoxical seizure, Drug withdrawal seizure, Diplopia, Psychosis (high dose), Toxic amblyopia, Encephalopathy, AV conduction disorder, Ventricular fibrillation, Nausea, Vomiting, Constipation, Diarrhea, Megaloblastic (folate-deficiency) anemia, Hypocalcemia, Hepatotoxicity, Hypertrichosis, Lymphadenopathy, Purple glove syndrome, Rash, Allergic reactions in the form of rash or, rarely, more serious forms (drug reaction with eosinophilia and systemic symptoms, or DRESS) or anaphylaxis, Purpuric rash, Toxic epidermal necrolysis, Bullous dermatosis, Coarsening of facial features,Periarteritis nodosa,Immunoglobulin abnormalities, Altered taste sensation, including metallic taste, Peyronie disease