There are quite a few studies available now that have researched the low effectiveness of anti depression medicine and the great difficulty in withdrawing from meds that artificially flood the brain with chemicals. As someone said above the success of AD is only in the 30% range. The more interesting question is why they work so well for that 30% and not for the rest.

For my part ADs were horrible and made my life much worse off and all I heard from Pdocs was that the side effects were in my head, side effects that are now accepted as real! "let's try this one or this combination" they didn't have a clue. They just played Russian roulette with my life. It wasn't until I grew a pair and said no more that things starting getting better. Eating right, quiting sugar and artificial stuff helped a great deal and when I did need medicine it wasn't ADs but mood stabilizers that helped.