Herpes around the mouth is caused by a virus, which is very common. Herpes can cause sore on the lips. You can also have the virus in your body without getting any mouth ulcers. Herpes usually passes by itself within a week and usually requires no treatment. There are prescription antiviral creams and patches that you can use on the mouth ulcer. If you have severe herpes disease or are susceptible to infection, you may have antiviral medicines on prescription.