@HaLeiVi – I’m not a speech therapist, I stutter so I know plenty about that area of speech therapy.
@PYid – Yes, Vitamin B1. I don’t know if I’m allowed to post links, but if you google “national center for stuttering preliminary research”, that top link is findings from Dr. Shwartz on B1 in adults based on a published study in kids from the 1950s.