Patients sometimes report a feeling of electricity in their jaw or lip when they are being given an injection in their lower jaw. Sometimes they report a burning or shock in their tongue. The injection most dentists use to numb the mandible is called a mandibular block. This injection is a nerve block of the mandibular nerve; a nerve that innervates the mandibular teeth. If it is pierced by the needle it will cause the patient to feel a burning sensation or a feeling of electricity in their jaw, teeth or lower lip. Anterior and adjacent to the mandibular nerve is the lingual nerve. The lingual nerve is responsible for the sensation of the tongue and if it is pierced the patient usually will report a pricking sensation or a burning on the tip of their tongue.
Although it is extremely disconcerting to patients experiencing it, this 'shock' usually causes no lasting harm and when their local anesthetic wears off their lip sensations return to normal.
If the mandibular nerve is damaged a patient can develop a lasting paresthesia of their lip on the side of the injection. Usually this paresthesia will eventually go away, but it may take 6 months to a year for lip sensations to go fully recover. Damage to the mandibular or lingual nerves is infrequent. I have been practicing for over thirty years and I have given many thousands of mandibular injections and I can count the number of paresthesias that have resulted from my injections on one hand. All eventually recovered the feeling in their tongue or lip.