For me, Ritalin (methylphenidate) actually improved my speech. I usually have a severe stutter but when I first took Ritalin I was talking away (and speaking really fast too). I asked my psychiatrist why that is, and he said that dopamine should, if any, improve psychomotor coordination, not worsen it. Dopaminergic drugs are in fact used to treat Parkinsonism and Bradykinesia and too much dopamine, in fact, causes psychomotor agitation (ie psychosis, extra pyramidal symptoms, etc). That's the theory he offered. These stimulants do cause anxiety and agitation and can worsen depression, so that may be what's causing/exacerbating the stutter.