The U.S National Institutes of Health says there isn't enough evidence to support the use of licorice for any health condition. But at least one study from Iran on glycyrrhizic acid, an ingredient in licorice, found that it may have some efficacy, but a lot depended on the incubation period of the virus.
If you'd like to give licorice a try, chew on a licorice whip. Just be sure it's made from real licorice, as most "licorice" candy in the United States today is flavored with anise. If the ingredient list reads "licorice mass," the product contains real licorice. You could also try buying some licorice powder and sprinkling it on the sore, or combine a pinch of licorice power and with a smidgen of pure vegetable shortening, then apply to the sore.
Remember, though, don't go overboard! Just as with any medicine, real licorice's medicinal effects can cause serious side effects if used in large amounts over long periods of time.