Every year well over 200 million prescriptions are handed out for antidepressants in the United States. Of the people who take antidepressant medications some will feel a certain level of relief and some won't benefit at all. So, if depression is simply due to an imbalance in neurotransmitters, why are some people ‘treatment resistant' when they take the very medication designed to correct the problem?

Professor Eva Redi, of the Northwestern University Feinberg School of Medicine, believes...