Numbness In Hands And Fingers. Using Meth. What Could Be Causing This?

Yes, hi. I have a query actually. I have a friend of mine his name is jake. he makes a living doing a lot of manual labor for a lot of hours, doing tree and lawn work. (cutting down huge tress, hauling them off, etc) He also does meth. not just smokes it or snorts it either. He shoots it up in hs arms, Now here's where my question comes in. For the last couple of weeks or so he's has had awful numbness in his hands and fingers. He still continues to use meth, and especially when hes doing his shot he says it gets a lot worse, that his entre hand and some of his arm goes numb sometmes. So, I'm thinking, hmm, could it be a nerve pinched or damaged in some other way? awhile back something similar happened to my b/f where his middle fingers were going numb and were numb almost all the time and he figured he probably poked a nerve when he was doing meth interveneously. and one day he woke up and it was gone. not numb anymore. just like that. and it is still to this day no numbness at all.. so can u tell me what is your take on this? what do you think his problem is? becuase he SAYS hes gonna go to the doctors and see but i dont know if i believe him. i do know he refuses to tell whoever his dr. is about the fact that he does meth. so any imput would help! i also have another question if you get back to me on this one. thanks!