Hello, Both of these medications help in inducing ovulation and hence used in the treatment of infertility associated with PCOD. Letroz works differently than siphene but has the same effect of stimulating the ovary to release eggs (ovulation). Siphene is a non-steroidal and ovulatory stimulant. It acts as a selective estrogen receptor modulator that helps in ovulation. Letroz is a non-steroidal aromatase inhibitor that works by decreasing the amount of estrogen produced by the body thus inducing ovulation. As compared with siphene, letroz is associated with higher live-birth and ovulation rates among infertile women with the polycystic ovary syndrome. Hence letroz is superior siphene as a treatment for infertility in women with the polycystic ovary syndrome. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr. Mohammed Taher Ali, General & Family Physician