On a GnRh agonist, you are chemically castrated which means testes stop producing any male hormones. The only remaining androgens (and DHT, produced from peripheral conversion of weaker, precursor androgens) comes from the adrenal glands. Studies have shown that post-op, there remains 40% of DHT in tissues (i.e. prostate) despite a 95% reduction in T in the blood (serum). Those very sensitive to DHT may perhaps benefit from finasteride but IMHO, it is probably overkill and unnecessary. You need to discuss this with your doctor.