Wide use of local anesthetics (LA) in dentistry determines high incidence of side-reactions to them. The development of complications connected with the use of LA depends on the general condition of the patient (age, the presence of hepatic, renal, cardiovascular diseases, or high drug sensitivity), the degree of the anesthetic's toxicity, its dose and blood concentration, peculiarities of its pharmacokinetics, the technique of administration, the degree of tissue vascularization in the site of injection, as well as the presence of a vasoconstrictor and additive agents in its solution. Practice shows that there is cross sensitivity to drugs belonging to different pharmacological groups, which should be taken into account when administering LA. A dentist should remember about possibility of serious side-reactions to LA, and follow recommendations on their administration (doses, contraindications, and interaction), as well as take into account allergy background and be prepared to take necessary emergency measures in the event of complications.