Oral sodium phosphate (OSP) for bowel preparation before colonoscopy has been widely prescribed for many years. However, its safety, especially the possible renal damage, gradually raised concern in recent years. Here, we report a diabetic patient on metformin, developing lactic acidosis and acute renal failure after taking OSP for colonoscopy. This patient recovered after emergency hemodialysis. OSPbowel cleansing may be contraindicated in patients on metformin for diabetes mellitus.