Hello there,
The most common side effects of Metformin are abdominal or stomach discomfort, diarrhea, lower back or side pain, muscle pain etc.The common causes of sore breasts are pregnancy, premenstrual syndrome and hormonal fluctuations. Dark areas of pigmentation on the face can be due to long-term exposure to UV radiation from the sun or artificial tanning devices. These areas of dark pigmentation are called solar lentigines and are also called liver, sun or age spots. Post-inflammatory hyper pigmentation, underactive adrenal glands all can produce areas of dark pigmentation as well. A physical examination may be necessary for correct diagnosis and treatment. A clinical examination will be helpful.
Best wishes and regards!