Sexual dysfunction and hyperprolactinemia are significant adverse effects, which may occur due to use of antipsychotic drugs. In this case report, our aim is to present sexual dysfunction due to hyperprolactinemia after administration of 150 mg/month IM paliperidone palmitate (PP). A 22-year-old, single, male university student was hospitalized with a diagnosis of psychosis. He was not using alcohol or other psychoactive substances at that time. Fasting blood glucose, lipid profile, complete blood count, thyroid function tests were all within normal range. The patient has never used antipsychotic drugs previously. We planned to begin PP treatment via IM route. Recommended initiation doses (150 mg day 1 and 100 mg day 8) were applied into deltoid muscle. In addition to that, 150 mg/day quetiapine was used. The patient did not describe any sexual complaint during hospitalization and he was discharged at the end of the three weeks with clinical remission. The treatment was continued with 100 mg/month PP injection for 3 months, and no sexual adverse effects were observed in the course of his follow-up. Psychotic symptoms were relapsed at the end of the third month; therefore fifth dose of PP was applied into the gluteal region as 150 mg/month. This dose was continued for two months. In this period; psychotic symptoms were regressed, but the patient complained about sexual aversion and erectile dysfunction. In this period; serum prolactin level was 60.98 ng/ml (normal prolactin interval: 4.04-15.2 ng/ml) and the patient was assessed with Arizona Sexual Experiences Scale and a sexual dysfunction was found in all subitems. Sexual dysfunction is a significant side effect of antipsychotic medications due to hyperprolactinemia. In a study assessing the potential changes in prolactin levels and sexual function after switching from long-acting injectable risperidone to PP, it was found that there was a significant reduction in prolactin levels and improvement in Arizona Sexual Experience Scale scores .It is mentioned that there is no dose-response relationship between paliperidone and prolactin. Also some other investigations have found no relationship between blood concentrations of paliperidone and related adverse events. In contrast to studies mentioned above, sexual dysfunction in our case occurred after increasing the monthly dose of IM Paliperidone Palmitate.