Local anesthetics work by diffusing from the injection site through surrounding tissues to reach the nerve membrane and block sodium channels. Placing the drug in the soft tissue just under the mucosa—the subcutaneous layer—puts it in close proximity to the nerve, allowing diffusion into the perineural space and onto the nerve fibers to produce numbness. This subcutaneous depot balances diffusion distance with adequate tissue containment, giving reliable onset and duration. Depositing into muscle would place the drug in more vascular tissue, leading to faster systemic absorption and less effective local blockade. Injection into bone (intraosseous) or directly into a vein is not how standard nerve blocks are achieved and carries different risks. So, most local anesthetics are deposited in the subcutaneous tissue surrounding the target nerve.

Local anesthetic solutions that include a vasoconstrictor like epinephrine are prone to oxidative degradation; preserving the vasoconstrictor’s potency requires an antioxidant. Sodium bisulfite acts as a sulfur-containing antioxidant that scavenges oxidizing species and helps keep epinephrine in its active form, preventing loss of effectiveness. Sodium bicarbonate is just a buffer and doesn’t prevent oxidation. Ascorbic acid is an antioxidant in some contexts but isn’t the standard additive in these anesthetic formulations due to compatibility issues. Glutathione is a strong intracellular antioxidant but isn’t used as a preservative in local anesthetic solutions. Therefore, sodium bisulfite is the antioxidant that prevents oxidative/biochemical degradation of the local anesthetic solution.

In systemic anaphylaxis, the immediate, life-saving intervention is epinephrine. It acts on multiple receptors to reverse the dangerous changes that occur during a severe allergic reaction: alpha-1–mediated vasoconstriction raises blood pressure and reduces swelling in the airways and tissues, while beta-2–mediated bronchodilation eases airway constriction and decreases mast cell mediator release. This rapid, broad action stabilizes breathing and circulation, addressing the two most threatening problems in anaphylaxis. Because of this, epinephrine should be given promptly, with a repeat dose if symptoms persist and emergency services contacted. Other options are not sufficient for the acute management. Antihistamines may help with itching or hives but do not relieve airway obstruction or hypotension quickly enough. Corticosteroids uptake is delayed and does not treat the immediate life-threatening symptoms. Simply observing and waiting risks progression to fatal airway compromise or shock. After epinephrine, provide supportive care such as oxygen, IV fluids if hypotensive, and airway monitoring, with continued observation for potential biphasic reactions.

The duration of anesthesia from a local anesthetic is mainly governed by how readily the drug remains in the nerve and how tightly it binds to nerve proteins. Bupivacaine is highly lipid-soluble and has strong protein binding, so it diffuses into nerve tissue and stays there longer, producing a longer-lasting blockade. This extended nerve effect translates into more prolonged postoperative analgesia compared with other common anesthetics. When used with a vasoconstrictor like epinephrine, its duration is further extended because blood flow is reduced, slowing the drug’s removal from the site. The other agents—lidocaine, articaine, and prilocaine—tend to have shorter durations due to less lipid solubility and protein binding, so they provide quicker onset but briefer postoperative pain relief.