Cyproheptadine prevention of the neuropsychiatric adverse effects of an antiretroviral regimen including efavirenz has been evaluated in a randomized clinical trial. Twenty-five patients (16 males and 9 females with mean±SD ages of 36±9 years) in a cyproheptadine group, and 26 patients (17 males and 9 females with mean±SD ages of 34±7 years) in a control group completed the trial. Sexual contact and injection drug use were the main routs of HIV infection in both groups. The patients' neuropsychiatric adverse effects were evaluated based on the Hamilton Depression Rating Scale, Hamilton Anxiety Rating Scale, Positive and Negative Syndrome Scale, Beck Depression Scale, Pittsburgh Sleep Quality Inventory, Positive and Negative Suicide Ideation, and Somatization Subscale of Symptom Checklist 90 at baseline and 4 weeks after treatment. Cyproheptadine significantly decreased the scores of Hamilton Depression Rating Scale, Hamilton Anxiety Rating Scale, Positive and Negative Syndrome Scale, Beck Depression Scale, Pittsburgh Sleep Quality Inventory, Positive and Negative Suicide Ideation of the patients after 4 weeks in comparison with control group. All of the scores increased in control group following antiretroviral therapy. Although short duration of the patients' follow-up was a major limitation of the study, the results of the study showed that cyprohepradine is effective in prevention of depression, anxiety, hallucination, aggressive behaviors, emotional withdrawal, poor rapport, poor impulse control, active social avoidance, suicidal ideation, and improved sleep quality of HIV-positive patients after initiation of antiretroviral therapy including efavirenz.