The important thing with serotonin, is to keep it at steady levels. The medicines that raise the level of serotonin in the brain do so by slowing the reabsorbtion of serotonin. The alcohol increases the availible serotonin for a bit and then it drops off quickly, leaving the depressed person feeling worse, and they tend to not take the medicine correctly when they feel badly or are drunk. High serotonin levels do not mean somebody will feel happy or good, It makes it more likely that they won't feel realy bad.
