Massachusetts senator Elizabeth Warren has asked the Centers for Disease Control and Prevention (CDC) to consider the role legal marijuana could play in the prescription opioid epidemic.Warren asked for more research into medical marijuana and painkiller addiction in a letter to the CDC director, Thomas Friedan."Opioid abuse is a national concern and warrants swift and immediate action," Warren wrote. Her request comes as politicians, including the presidential nominees, search for the best response to the opioid epidemic. The use of prescription opioids doubled between 2000 and 2014, according to the CDC. And Massachusetts experienced its highest number of unintentional opioid...