the only opioid I've tried is tramadol and it made me so relaxed I became almost fluent. Stimulants on the other hand (amphetamine, mephedrone, mdma) most of the time made it harder for me to talk, something to do with my neck and voice chords feeling more "locked" and constrained. But this was inconsistent because sometimes I would actually stutter less. Stuttering is a weird neurological disorder that's for sure