Have a white sore that has been persisting under my tongue for a week now. Have taken folic acid tablets & multivitamins but no help. Please advise.

Need more info. Do you smoke or do beatle nut ? Or masala? If yes stop it right away.If no consult your doctor or dentist. You may have a sharp or broken tooth causing trauma. Persistent sores over 3-4 weeks could be precancerous , hence do not neglect and take expert opinion early. Better safe than sorry.
Family practice . You probably have a canker sore caused by the herpes virus. It can last for several weeks. Usually painful. Hydrogen peroxide is helpful by dabbing on it several times a day. Anti viral medications are used but need to be started early on.