Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Zytiga is composed of abiraterone, which works by reducing androgen production in the body. Androgens are male hormones that can promote tumor growth in the prostate gland. Zytiga is used together with Prednisone to treat prostate cancer that has spread to other parts of the body. Since your PSA levels seems to be fluctuating, I would advise you to get evaluated by consulting your doctor as to know what could be the cause. You may need few investigations to confirm the diagnosis. Hope I have answered your query. Let me know if I can assist you further.