Preventive antibiotics are frequently prescribed by dentists for older patients with certain heart conditions, but Oregon State University researchers recently found that those antibiotics, which can expose patients to unwelcome side effects, are unnecessary 81 percent of the time. Using four years of a healthcare claims database of almost 170,000 prescriptions involving more than 90,000 patients with an average age of 63, the research found that fewer than 21 percent should have been given antibiotics based on heart conditions. The findings are important because dentists are responsible for 10 percent of all antibiotic prescriptions written in the U.S., and overuse of antibiotics contributes to bacteria evolving to make the drugs ineffective.