How to use Cymex Ultra, a cold sore treatment It’s best to apply Cymex Ultra to cold sores as soon as possible, preferably at the first sign of their arrival but treatment can be started later if necessary. Apply a thin layer of cream to the affected area five times a day at Approx. four hourly intervals, to help get your lips back into tip-top shape with our cold sore treatment. Continue for at least 4 days. If your skin infection does not get better after 10 days of treatment, tell your doctor as soon as possible.