Welcome to ATD, thank you for the query.
Metformin doesn't change the colour of urine, it is the amount of water you drink that does. If you don't drink enough water your urine may be darker.In a study, only 0.03% people on metformin had urine colour abnormal.
Do inform your doctor about the change in colour of urine as he will be in a better position to assess the reason for the same.
Hope this helps.
All the best.
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