Local anesthetics, or LAs, often diffuse into the systemic circulation and affect the organs that depend on the functioning of sodium channels. The central nervous system is typically most affected, followed by the cardiovascular system. The extent of the adverse effects depends on the LA concentration in the bloodstream. At low systemic concentrations, LAs induce dizziness, and auditory and visual disturbances. They also numb the tongue and leave a metallic taste. As the serum concentration of the LA peaks, the untreated initial symptoms progress to cause tremors, convulsions, respiratory arrest, vascular collapse, and even death. Additionally, some individuals are allergic to LAs and suffer from allergic dermatitis or asthma. Most allergic reactions are caused by the metabolism of ester-linked LAs, which yields the allergen para-aminobenzoic acid. An allergy from amide-linked LAs is rare but can occur due to the presence of preservatives like methylparaben.