Hi I understand your concern. Mirtazapine is antidepressant and it increase your apatite to some extent and there is weight gain occur. But its role in cancer is questionable?/ Prostate cancer with mets require chemotherapy and radiotherapy. Weight loss is the main features of any cancer. So Go for medical assistance as you did not have till date. It is surprising so please take consultation to cancer specialist and get help. Earlier the treatment better the response. hope for the best. Still have a query then feel free to ask. Happy to help you. Take care.