Millions of people with stents that prop open clogged heart arteries may need anti-clotting drugs much longer than the one year, doctors now recommend. A large study presented Sunday found that continuing an aggressive course of blood-thinning drugs for an additional 18 months lowers the risk of heart attacks, clots, and other problems. In the study, which involved nearly 10,000 patients, those treated with aspirin plus a second anticlotting drug for 30 months had a sharp reduction in the risk of heart attacks and other complications compared with those who stopped one of the blood thinners after a year. Stents are tiny wire-mesh devices that are implanted to prop open diseased coronary arteries. They are commonly used to treat patients suffering a heart attack or chest pain called unstable angina.