Is It Safe For Me To Use Metformin For Acanthosis Nigricans With Borderline Sugar Levels?

Hi Iam Male 31, I have dark patches on neck and under arms for which my doctor has confirmed as Acanthises Nigericans. She has prescribed me Metformin 500mg twice a day and 5 times exercise. My sugar levels are at border like FBS is 100. She also prescribed Cream contains Glycolic acid to apply at night. My questions is is it safe to use Metformin in my case and iam worried that I should use metformin though out my life. Is that so? Iam in the process of loosing my weight and started 2 months ago and getting good results. Iam 5.11 and weight 120Kgs (264lbs). Please provide your views. If we start metformin should it be continued forever? and glycolic acid cream is it fine to apply over the affected areas (neck). Thank you