Is it safe to take metformin while pregnant or breastfeeding?

Very safe. Metformin has been looked at and found to be very safe during breastfeeding. A small amount does enter the milk, the relative infant dose is 0.3-0.7% of mom's dose. It is considered a level 1, the safest category for breast feeding. See:hale, t;medications and mothers milk, hale publishing, 16ed, 2012, pp744-745.
No. Metformin is an oral medication used to treat diabetes and pcos (polycystic ovary syndrome). At this time, there are no studies that document metformin's safety in pregnancy or nursing. Current recommendations are to use alternative medications.

Related Questions

Is it safe to take metaglip (glipizide and metformin) while pregnant or breastfeeding?

No. Of course, only your obstetrician can answer this question for your particular circumstances...But in general, diabetes (either gestational diabetes or previously acquired diabetes) during pregnancy has better treatment options than metaglip (glipizide and metformin). These treatment options include other oral medications, Insulin therapy, and, in some cases, simple dietary management. Read more...
Category C / L3. This medication is considered a category c drug, meaning animal studies suggest some risk but no human studies to date show adverse effect. L3=lactation studies show little of this drug makes it into bm= .6% so it is considered relatively safe. See:hale t;medication and mothers milk, 16ed, 2012. Read more...

Is it safe to take metformin for pcos while breastfeeding my 8 month old?

It is safe/increases. All evidence shows that taking metformin and brest feeding is safe.It can also increase the production of milk with mothers with pcos. Read more...
Yes. Only 0.28% of the weight-normalized maternal dose of metformin enters human breast milk. In rodents, the metformin concentration in milk approaches that of the maternal plasma. This is well below the 10% level usually expected for the concentration to have a clinical impact. Please do not attempt another pregnancy until the first birthday of your little one, though. Take Folic Acid 1-5 mg daily! Read more...