Concern is growing about the overuse of antibiotics and the subsequent rise in antibiotic-resistant bacteria. Antibiotics are commonly used to prevent heart valve infections in susceptible patients undergoing dental or other surgical procedures. Although this practice has been standard for nearly 50 years, little evidence exists that it works. This Issue Brief summarizes a population-based study that challenges the link between dental procedures and heart valve infections, and illustrates the difficulty in incorporating new evidence into existing guidelines and longstanding practices.