Is it safe to take Metformin for PCOS?

I've been suffering from PCOS since last 5yrs and I'm 23 now. Last year during ultrasound I was detected with Ovarian Cyst with 6 cm. And before 6 months I was given Metformin for 3 months. While I was on Metformin 500mg 2 times per day for 3 months I had my periods regular but after stopping it I again started to have irregular periods.But I started to have my back bone ache. I had an ultrasound the day before and the report was I had 8cm Ovarian cyst. And the gyno suggested me Metformin 500mg 3 times per day for 3 months.I'm unmarried and I want to have baby in my future. Is it safe to take Metformin like this?