I do both. I always have, but methylphenidate (without accompanying TCA), Adderall (but not Dexedrine), and Wellbutrin make me do it a LOT worse.
Once, I accidentally bit my lower lip so hard, I severed the salivary duct & gave myself a mucocele. Ouch. I still remember the bite, but not the reason. I just remember that something made me bite down really hard by accident, and the blinding pain afterward made me realize that this wasn't just an ordinary accidental bite, and that something really bad had just happened that never happened before.
I tend to bite my lip (gently) in situations when I feel like I'm not really sure WHAT kind of facial expression I'm supposed to have. I'll kind of curl my lower lip over my lower teeth (like an old woman without teeth), and gently bite down (making contact, but not applying actual pressure). My "stress" bites tend to involve the left or right side of my lower lip, and are completely different from my "I'm mystified" bites.
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Your Aspie score: 170 of 200 · Your neurotypical (non-autistic) score: 34 of 200 · You are very likely an Aspie [ AQ=41, EQ=11, SQ=45, SQ-R=77; FQ=38 ]