The food and drug administration and the governments agency for healthcare research and quality are embarking on a large-scale study of ADHD medications. The study will include data on about a half million children and adults whove taken drugs to treat attention-deficit hyperactivity disorder. All drugs currently on the market for ADHD will be studied. Researchers aim to find out whether the drugs increase the risk for heart attacks, stroke or other cardiovascular problems. It is expected to take about two years.