The functionality of a meth head depends on the quality and availability of their junk. Some carry bibles, some need it to sleep, some you'd never know unless you know what to look for. I've seen guys turn into some weird sumbitches on that crap.
I grew up in an area riddled with meth heads, dealers, and bathtub cooks. My own parents and brother were meth heads and dealers at one point. I've literally had no contact with my dad and brother for going on 19 years now. My mom has been clean for a long time now, but I doubt they are.
Thanks to meth I haven't seen one single family member in person for 15 years now. As far as I'm concerned they can round up all the meth heads and shoot them fuggers to the moon.
Sorry about the rant folks.