A few days ago, a client asked me how she could tell when she was hungry. "I know what it is like to be thirsty," she said. "But I am not sure I can tell the difference between being hungry and just wanting to eat. After I started on antidepressants I found myself putting food in my mouth even though I don't think I should be eating."

I told her that she had pinpointed one of the biggest problems facing people on medications like hers, along with anyone concerned about weight. We seem to...