Ten years ago, a sharply divided Congress decided to pour billions of dollars into subsidizing the purchase of drugs by elderly and disabled Americans. The initiative, the biggest expansion of Medicare since its creation in 1965, proved wildly popular. It now serves more than 35 million people, delivering critical medicines to patients who might otherwise be unable to afford them. Its price tag is far lower than expected. But an investigation by ProPublica has found the program, in its drive to get drugs into patients' hands, has failed to properly monitor safety.