We now know that the serotonin theory of depression (which essentially says that not enough serotonin = depression) is overly simplistic, incorrect, and doesn't even come close to explaining the complex and multifaceted picture that is depression. In fact, we have known this for decades. (Take a look at this very brief article that outlines various sources of evidence of this.)
This has to make us wonder why psychiatrists and the pharmaceutical industry continue to perpetuate this theory and push serotonin-related antidepressants (i.e. SSRIs) on so many people...
The reality - and one that many people are surprised to learn - is that antidepressants have a very low success rate. Actually this turns out to be pretty unsurprising when you learn that SSRIs are targeting something that doesn't cause depression in the first place.
So what actually works to treat depression? A combination of reducing systemic inflammation throughout the body, healing the gut, correcting nutritional imbalances, eating a nutrient-rich diet, addressing hormonal imbalances, exercising, managing stress, and talk therapy. Depression is a symptom of imbalance and inflammation within the body, and the way to "cure" it is to correct the imbalances and significantly reduce systemic inflammation.
I specialize in the treatment of depression, so please contact me if you or anyone you know is suffering. It makes me incredibly, incredibly sad to see so many people suffer unnecessarily for years on end because they are getting ineffective, outdated treatment. I use evidence-based methods that work to correct the underlying imbalances and systemic inflammation in the body that cause the symptoms of low mood, irritability, feelings of worthlessness, and so on. Once we address the real culprits leading to depression, the depression itself goes away.