The FDA have not approved metformin for weight loss purposes. However, some doctors will prescribe it to people who are overweight or obese and have type 2 diabetes or prediabetes.
That said, if metformin is prescribed to someone who meets these conditions, that person needs to adopt other healthful habits to support weight loss. These habits include following a nutrient-rich, low-calorie diet and getting regular exercise.
Metformin is not a quick-fix, miracle-diet drug. It is much more likely that people taking metformin will likely lose weight gradually over a long time.
It is also worth noting that people who lose weight while taking metformin may regain it if they stop taking the drug.