A recent report came out that some metformin products in Singapore may contain NDMA above accepted daily intake levels (96 nanograms).
NDMA is classified in group B2 by the EPA. "Probable Human Carcinogen"
There is inadequate evidence that it can cause cancer in humans but at present it is far from conclusive.
Until the FDA, Health Canada, EMA and other regulatory agencies find otherwise with testing, the use of Metformin is still encouraged as "The risks from not having adequate diabetes treatment far outweigh any possible effects of the low levels of NDMA seen in metformin medicines.” "The levels of NDMA seen in the affected metformin medicines are very low and appear to be within or even below the range that people would normally be exposed from other sources, including food and water."
So as of right now the risk is very low.
If you have any concerns, contact your MD.
**Update Feb 26 2020
There is now a recall in effect