No, do not shoot up in your mouth, listen to muvolution. You want it to be as sterile as possible and mouths are not sterile, plus.. just no, would be extremely difficult and pretty painful I imagine!
I really can't see the HR in this. Anywhere other than your arms and you are getting progressively more risky - there are threads in OD about locating veins, but BDD's not the place for a thread about comparing crazy/dangerous shooting up stories I feel.. check the guidelines in my sig.