I take berberine instead of metformin, since I can't tolerate one of the inactive ingredients in metformin and I can't afford to get it made for me without that ingredient.
What I found was that berberine didn't work for me by itself, but when I added black seed oil, it gave me blood sugar that was suddenly responsive to my efforts. I was taking that for a couple of weeks before my hbA1c of 55.2 came back, and I wasn't seeing much result. But that number scared me into trying again with walking and lowering my carb.
In the weeks before I started taking berberine, I had been trying walking and lowering my carbs ever lower. It all went for naught. Walking made my blood sugar rise. Lower carb, gave me highs as well. None of it made sense. But now, on the herbs, I started seeing the results I had expected.
I'd be cautious about adding berberine to a pharmaceutical without my doctor monitoring it, though. Interactions could be very unfortunate.
(Oh, and at first berberine nauseated me terribly. I found that a capsule of neem leaf taken at the same time helped a great deal. Neem isn't recommended for long term use, so I stopped after one bottle, and the nausea didn't come back.)