Dental abscess. Close-up of an abscess in the mouth of a dental patient. A dental abscess is a painful pus-filled swelling caused by a bacterial infection in the tissue around the root of the tooth. A dental sinus (also known as a parulis or gumboil) may form in the gum, where the dental infection drains onto the surface. A dental abscess can be treated with antibiotics, but may also require drainage, root canal treatment or tooth extraction.