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Diabetic help: What should be the alternative way to do when metformin no more works? - metformin more drug_uses

I am over 10 years with diabetes.
I used to take 1 tablet containing 500 mg metformin with food, 3 times a day, and my blood sugar under good control.
Lately my blood sugar is high, the reading of 13 and 16 mmol 2 hours 2 tablets of metformin with food.
Each complementary medicine for me, or herbal recommend?
Thank you.