This study aims to explore whether short-term measures of eating behavior can be related to later, longer-term weight loss. In a double-blind, randomized fashion, participants will be asked to take study medication for three weeks: one week of phentermine and two weeks of placebo. At the end of the first and third weeks, participants will be asked to participate in a range of eating behavior studies. Following this three-week, double-blind phase, participants will be offered 6 months of open treatment for weight loss.