Milk Thistle is not one of the herbs known to readily interact with other medications. Some herbs have a specific ability to alter the way the liver processes medications which can either raise or lower blood levels of that medication.
Because milk thistle is an herb that acts on the liver there is the possibility that it may alter the levels of metformin in your blood thus affecting your blood sugar. What I suggest is taking the Milk Thistle at least 2-3 hours away from the Metformin. In addition, start with the smallest dose possible (i.e. 1 capsule). Do this for about 1 week while regularly monitoring your blood sugar. If you do not notice any significant difference in your body’s blood sugars, increase the dose. Continue to do this process over the course of several weeks until you have reached your desired dose.
Even though I would not expect an interaction between the two, everybody’s body is different and it is best to error on the side of caution when combining herbs and medications. So, just remember to take it slow, and start low and you should be just fine.