As someone who had taken numerous anti depressants, I'm not at all convinced about the chemical imbalance theory. The anti depressants I've taken have only ever given me (quite serious) side effects without actually doing anything for the depression or my Co morbid metal illnesses . The most annoying thing is psychiatrists (the ones I've seen) completely buy into the chemical imbalance theory and seem surprised when a specific anti depressant doesn't work or they almost don't believe you.
Depression and mental illness seems such an unknown and mysterious area of medicine and I've came to the conclusion after 20+years of various treatments, science just doesn't have the answer at this moment in time. I'm at the point now of believing that for me anyway , being on anti depressants is just as bad as being off of them,2 sides of a horrible coin.