Hello, Thank you for your question. The common side effects of metformin are gastrointestinal, including a metallic tasts in the mouth, mild anorexiaa, nausea, abdominal discomfort, and soft bowel movements or diarrhea. These symptoms are usually mild, transient, and reversible after dose reduction or dicontinuation of the drug. Although metformin is generally well tolerated, patients taking metformin may develop gastrointestinal side effects even after many years of use. If gastrointestinal symptoms develop while taking metformin, we institute a metformin holiday, which may lead to resolution of symptoms. after a period of non use, metformin may be successfully resumed at the same or a lower dose with slow titration of the immediate-release or extended release formulation. You can let him stop taking metformin. And go to see his doctor why he prescribe metformin for him. Hope I have answered your query. Let me know if I can assist you further. Best regards, Dr. Heang Chan Raksmey, General and Family Physician.