depression
"I've suffered from chronic depression for over 20 years.  I've been on a number of antidepressants and nothing has worked long term. I was put on this drug to ease the pain of a back injury and quite surprisingly and unexpectedly my depression symptoms disappeared completely. I became a happy productive individual overnight. Amazing!  Too bad it's an addictive opiate, and I imagine that, over time, the benefit would wane."