I've been using quite a few drugs during the last 2 years but honestly none of them affected my stutter in any way. Benzos just make me not give a fuck that I stutter anymore but they don't reduce it. I suspect your stutter is heavily dependant on your current level of anxiety and getting clean off stuff like meth, opiates and benzos is gonna definitely negatively impact your stutter. Honestly I don't even know what to suggest, other than carefully tapering off benzos, seeking a speech therapist and just waiting out a few years to see if it gets better.. Best of luck in your recovery