Correct me if I am wrong but doesn't amphetamine affect serotonin and dopamine levels? Studies has shown that stutterers have different dopamine levels than non-stutterers. (http://en.wikipedia.org/wiki/Stuttering#Dopamine_abnormalities) Couldn't amphetamine be responsible for changing the chemistry in the brain thus increasing stuttering; with anxiety as a side-effect of course. I am having trouble seeing anxiety being anything other than a contributing factor rather than the cause of the problem.