Yes, caffeine blocks adenosine receptors, which in turn causes a surge in dopamine. People who stutter already have an imbalance of dopamine in their brain, which has been linked to one of the causes of stuttering. If dopamine surges even higher, this can adversely affect fluency. My speech gets much worse with caffeine so I’ve cut it (and sugar) out completely, and my fluency has improved. Feeling a lot less jittery and anxious too. I’ve replaced my morning coffee with lemon balm tea or chamomile tea for anti-anxiety calming effect. Even green tea has caffeine, so be careful with that too.