Reading this article made me feel a lot better. Not only do we not 'run out' of Serotonin, but when levels are low, we can help to increase it.
Or, perhaps, by the time their levels are starting to become normal again, more substances are taken/smoked, putting it straight out of order again and their brain then has to start all over. I don't know. But it makes sense.
IMAGE source unfound. The image provided is not my own but I felt it fit the theme.