At Discover Magazine, psychologist Irving Kirsch argues that the current generation of anti-depressants don’t really address specific chemical imbalances.  He argues that they function at all because they are placebos:

[Current anti-depressants such as Prozac] all have different and in some cases opposite effects on brain chemistry, and yet they all show exactly the same response rate. It’s uncanny! That suggests it’s really the placebo effect that is helping the patients. In practice, all the different antidepressants have the same response rate. In a population of depressed people, they all work equally well. If they were actually correcting chemical imbalances, it would mean that the exact same number of people who are depressed have each kind of chemical imbalance: The proportion of people who have too much serotonin is exactly the same as the fraction who don’t have enough norepinephrine. The odds against that are astronomical.

I hope this makes it clear that I’m an equal opportunity skeptic.  I don’t merely pick on alternative, fringe and fraudulent treatments (e.g., homeopathy).