Depression, like cancer, probably has multiple different causes for different people. Imbalace in neurotransmitters like serotonin and GABA has been linked to presence or absence of some gut microbes, and glutamate likely plays a much larger role than previously suspected. The blind focus on serotonin alone, and push for SSRIs over the last few decades is a major disservice to the countless people they have failed to help. If they were truly effective, suicide and depression wouldn’t be among the top 10 causes of death and disability.