The Lantus is almost identical to human insulin that you produce yourself - some have a mild reaction to the preservatives in it (typically irritation at the injection site) and if you consume too many carbs while taking insulin it results in weight gain

Metformin causes a number of GI Issues but for the majority, those go away after a couple of weeks

Now IF you have some pre-existing kidney issues, lowered function etc then it must be monitored if you are taking Metformin

Metformin does NOT HURT the Kidneys BUT if it is not properly filtered out it can rise to toxic levels in the blood

Your Kidneys need to be functioning well enough to remove the Metformin "leftovers"

So doesn't hurt the kidneys, but if they don't work well enough you can't take the drug because it would cause some "other problems" if your kidneys can't clear it

The referral to the Kidney Doc (Nephrologist) could be to confirm your kidneys are healthy enough to handle the meds OR perhaps the Doc is worried about your kidneys and wants an assessment

Many diabetics have or will develop some stage of kidney disease (myself included - I was Stage 2 CKD "before" I was diagnosed with Diabetes and since diabetes is hard on the kidney's, while I'm still Stage 2 eight years after Type 2 diagnosis I do have my kidneys checked twice a year with two different tests each time to make sure they aren't getting worse

The main side effects is lower blood glucose levels.