The case of a 35-year-old female patient who was diagnosed as schizophrenia treated with psychotrophic drugs nearly for 15 years is presented here. After the disease was diagnosed, the patient quit her university education and began to live inactively far from her social environment, usually spending lazy time at home. During this period, due to either the effects of drugs which have to be used on hormones affecting appetite and body weight or her decreased physical activity, her body weight increased by nearly 30 kg. Anthropometric measurements, biochemical parameters and food diaries were evaluated at the beginning of the nutritional counseling and then repeated periodically. Upon obtaining biochemical findings, collaboration with other units started. The patient was educated on nourishing healthy and controlling body weight, also to bring about lasting behavioral changes. At the beginning of the therapy, among the biochemical measurements, insulin resistance was defined and metformin treatment was begun. Metformin therapy contributed to the patient's adaptation to the diet and improved glucose tolerance. In this way, it was possible to cope with the insulin resistance caused by anti-psychotic pharmacotherapy (clozapine) and the obesity which had developed as a result of clozapine. During the 18-month therapy the patient lost 27 kg, her body fat was reduced by 10% (18 kg) and BMI returned to normal levels. It is known that, many medications used in psychiatric disorders affect appetite and body weight. As seen in our patient metformin therapy causes weight loss and decreases insulin resistance. Both the illness and the medications used for treatment could affect the hormones which play a part in controlling body weight and the cytokines, as a result could change food preference and eating behavior which ultimately pave the way to obesity.