For serotonin syndrome, it doesn’t necessarily mean I have higher levels of natural serotonin. But my susceptibility to SS could be due to a deficit in SERT(An animal study by Fox and colleagues showed that SERT-deficient mice exhibit increased susceptibility to serotonin syndrome-like behavior when given serotonergic drugs.) So maybe I just don’t have enough transports so even a tiny dose of an ssri blocks too many transports. Did you see the link to the study I posted under someone’s comment? And I’m not implying that too much serotonin is the cause of stuttering for everyone, as it is a complex disorder, but it could be making the stutter worse for some people.