Mental Retardation (MR), is a well known brain disorder, which emerges early in life with many situations in which mental capacity is inhibited. But it is also a fact that, the patients are usually experiencing additional problems while being threatened for these problems, because of high dosed prescriptions of medically potent drugs . In this article, we aimed to discuss the clinical features, the progress, the differential diagnosis and treatment of a patient with mental retardation, who has tardive dyskinesia under long term use of a densely crowded set of pharmaceuticals. A 25-year-old single male patient was admitted to our clinic with the complaints of sudden onset, dysrhythmic, quick, unstable and aimless rough body movements, grimacing lips, body curling like a snake and restlessness for the last year. In mental examination, there was increase in molar activity, distractibility, dysarthric speech; he could understand simple sentences but had difficulty in simple calculations. The patient had been treated with various antipsychotics and mood stabilizers due to behavioral abnormalities since 2010; in 2012, abnormal involuntary movements had started after a 10 days treatment of carbamazapine 400 mg/day, quetiapine XR 400 mg/day and diazepine 5 mg/day. There was no pathology in the tests for differential diagnosis like cranial MRI, genetic analysis (Huntington corea), EEG, complete blood count, hepatic and thyroid functions, electrolyte levels, serum antibody levels, urine copper and ceruloplasmin levels. Treatment has been reorganized as valproate 1000 mg/day, E vit 800 IU/day and clozapine 25 mg/day to be increased gradually. There was a reduction in choreoathetoid movements 4 weeks after initiation of treatment. At the end of the fourth month, abnormal involuntary movement scale score decreased from 31 to 25, Rockland Simpson tardive dyskinesia assessment scale score decreased from 78 to 53 and Barnes acathisia scale score decreased from 14 to 12. Tardive dyskinesia (TD), which is late onset of using antipsychotics, has not been an effective treatment yet and causing permanently disabling sequela, is a complication due to the use of antipsychotic drugs. In our case, it caused TD that there was mental retardation on the ground, the patient’s age at first neuroleptic exposure was small and neuroleptic drugs were used at high doses for a long-term.The main purpose of us for presenting this case is to remember that the patients are more likely to have extrapyramidal symptoms in case of congenital abnormalities and organic brain syndromes and to underline that inessential or multiple drug use might cause more complicated situations.