I am an asthmatic and I would like to know if i can take Metformin?

I have a 5 year old daughter and my husband and I are trying for a baby, we have been trying for some time now but are having no luck – I was diagnosed with PCOS after having my 5 year old.
I have PCOS and would like some information on Metformin – As a few doctors have suggested it.
I am an asthmatic and I would like to know if i can take this medication while taking my asthma prevention?
Are there any long term side effects?
What dosage is normal?
How long do you normally take it for?
Can a any GP prescribe me this?

Age:24
Female
Medications: Singulair