Secondary tooth decay under old fillings is a silent issue that often goes unnoticed. Surgeon Sevak Barsegyan warns that since fillings typically last only 5–7 years, they eventually wear down and form microscopic gaps. Bacteria enter these crevices, destroying the tooth from within. If you experience sensitivity to sweets or cold, the decay may have already reached the nerve. Experts recommend dental checkups every six months to catch issues before root canal treatment is required.