Short version: they don't work better than placebos.
Right about here, people scowl and ask how anti-depressants--especially those that raise the brain's levels of serotonin--can possibly have no direct chemical effect on the brain. Surely raising serotonin levels should right the synapses' "chemical imbalance" and lift depression. Unfortunately, the serotonin-deficit theory of depression is built on a foundation of tissue paper. How that came to be is a story in itself, but the basics are that in the 1950s scientists discovered, serendipitously, that a drug called iproniazid seemed to help some people with depression. Iproniazid increases brain levels of serotonin and norepinephrine. Ergo, low levels of those neurotransmitters must cause depression. More than 50 years on, the presumed effectiveness of antidepressants that act this way remains the chief support for the chemical-imbalance theory of depression. Absent that effectiveness, the theory hasn't a leg to stand on. Direct evidence doesn't exist. Lowering people's serotonin levels does not change their mood. And a new drug, tianeptine, which is sold in France and some other countries (but not the U.S.), turns out to be as effective as Prozac-like antidepressants that keep the synapses well supplied with serotonin. The mechanism of the new drug? It lowers brain levels of serotonin. "If depression can be equally affected by drugs that increase serotonin and by drugs that decrease it," says Kirsch, "it's hard to imagine how the benefits can be due to their chemical activity."