MODEL RELEASED. Toothache. Woman holding or rubbing the side of her face to relieve the pain caused by toothache. Toothache is in most cases the result of tooth decay (dental caries). This is caused by bacteria which remain as a plaque on the teeth and which produce an acid that slowly destroys the tooth. Toothache occurs when decay is so advanced that the bacteria infect the living pulp inside the tooth, causing pain. Dental treatment involves drilling out the affected area and inserting a filling into the tooth.