DENVER - Tammy Quist, principal of Fairview High School in Boulder, was not surprised when she learned in April that a student was in a coma after overdosing on an over-the-counter cold medicine. "It's out there, and you know kids do it," Ms. Quist said. "It's cheap, accessible and it's not illegal if kids get caught. And it usually goes unnoticed until a kid takes too much or combines it with something to get a horrible reaction and overdoses." Cough and cold medications containing dextromethorphan, or DXM, are becoming increasingly popular among teenagers and young adults looking for a cheap...