Can Taking Metformin And Letrozol Be Safe ?

dear doctor, i am diagnosed with pcos. My age is 30 yrs. I am lean and my weight is 49.7 kgs. I am married in oct 2007 and trying to conceive for past 2 years. I have done ovarian drilling in dec 07. Then at present in treatment for induced ovulation. I am given clomiphene citrate and 3 iui already done with no success. This month i have been suggested to take metformin along with letrozol, i am totally confused. I am with clomiphene for past 6 months. Please reply