At 25 mm, your gallstone is quite large. While oral medications like ursodiol exist to dissolve cholesterol gallstones, they are only effective for very small stones (typically under 10 mm) and take months or years to work, with a high chance of the stones returning. Unfortunately, medication will not be effective for a stone of your size. If you are experiencing symptoms like abdominal pain, nausea, or bloating, the standard and most effective treatment is surgical removal of the gallbladder (laparoscopic cholecystectomy). I highly recommend consulting a general surgeon to discuss the safest treatment plan for your situation.