Experts who wrote new guidelines for preventing heart attacks and strokes are defending a formula that some doctors say overestimates risk for certain groups. American Heart Association experts say that any flaws in the formula are small and should not delay implementing the new advice. That advice greatly expands how many people should consider taking cholesterol-lowering statin drugs. The group’s comments came at its annual conference in Dallas after a newspaper report featured criticism by several prominent cardiologists. Statins are among the most widely prescribed drugs in Canada and the US. More than 38 million prescriptions for statins were filled at Canadian pharmacies in 2012.