For a 20 mm gallstone, treatment depends on whether you are experiencing symptoms. If you have no pain, we typically recommend simple observation, as immediate action isn't necessary. However, if you experience symptoms like abdominal pain, the standard and most effective treatment is a routine, minimally invasive surgery to remove the gallbladder (laparoscopic cholecystectomy). While oral medications like ursodeoxycholic acid can dissolve stones, they are generally ineffective for larger stones of this size and are reserved for those unable to undergo surgery. Rest assured, this is a very common and highly manageable condition. Please consult a doctor to discuss the best approach for you.