SWIM dont have an answer for ya, just SWIMs own experience. SWIM used to IV black tar (on West Coast of USA), clean for 2 years thanks to methadone, but definitely felt the feeling you are speaking of. It started burning (in veins) once fent was used. Exact feeling youre speaking of happened once when the consistency was very very hard, it was definitely a bad cut or something.... all head neck and tongue tingled burned SO bad. thought someone tried to give a hotshot. SWIM thinks your right on the money.