Maybe low b.p. and lack of oxygen in blood to feed nerve endings. . if i hold a joint or cigarette in my lips they can tingle like little needles . Same with tonge if i push on gums or tooth while salivating on sativas. But then i got acute toxic phenol poisening turned m.s.a. which i beat ,lol. I have no resiliancy ,lol. Other than that if a strain dont numb my lips i wont keep it ! Good shit makes my mouth muscles not work right at times. I may blow a " word " threw my lips because they wont work .lol. Then if super fucking good t.m.j. sets in with ears a ringin ;loud as fuck. lol. i know 't.m.i. " , lol