Many options to deliver anesthesia have developed over the last several decades. Administration of topical anesthetics to control pain associated with procedures such as laceration repair may avoid the need for infiltrative local anesthesia injections and associated pain from the injections.  Topical anesthesia also avoids the risk of wound margin distortion that exists with infiltrative injection administration. Many dosage forms exist (eg, gels, sprays, creams, ointments, patches) and provide the clinician with precise options for application under various circumstances.
Mechanism of Action
Topical anesthetics reversibly block nerve conduction near their site of administration, thereby producing temporary loss of sensation in a limited area. Nerve impulse conduction is blocked by decreasing nerve cell membrane permeability to sodium ions, possibly by competing with calcium-binding sites that control sodium permeability. This change in permeability results in decreased depolarization and an increased excitability threshold that, ultimately, prevents the nerve action potential from forming. [2, 3, 4]
Skin absorption is highly variable. Most anesthetic agents exist as solids and are only superficially absorbed through intact skin. Eutectic mixtures result in liquids that melt at lower temperatures than their single components. This permits higher concentrations of anesthetics, which results in superior dermal anesthesia for intact skin. Other methods of increasing skin penetration include liposomal preparations, iontophoresis, and transdermal patches. [2, 4, 5]
A recent double-blind, paired study of 82 adult volunteers compared a lidocaine/tetracaine transdermal patch (Synera) with lidocaine/prilocaine cream prior to vascular access at the antecubital site. The lidocaine/tetracaine patch provided effective anesthesia in as little as 10 minutes and results showed superior anesthesia at all application times less than 60 minutes. 
Topical anesthetics are used for various skin and mucous membrane conditions, including (but not limited to) pruritus and pain due to minor burns, skin eruptions (eg, varicella, sunburn, poison ivy, insect bites), and local analgesia on intact skin. With the exception of lidocaine-prilocaine as a eutectic mixture (EMLA), topical anesthetics are poorly absorbed through intact skin. Because of variation in systemic absorption and toxicity, the ideal choice of topical anesthetic and particular concentration depends on the intended use. EMLA has also been applied to children to minimize discomfort prior to injections or to starting an intravenous line. [3, 4, 7]
Dosage Guidelines and Administrative Techniques
See the list below:
Apply the cream, ointment, or solution to the chosen area in incremental amounts.
The total dose for a topical anesthetic is smaller than that used for subcutaneous infiltration.
Sequential layered application of topical lidocaine with epinephrine (TLE) has been described for small facial or scalp wounds. One-hundred patients were enrolled in a randomized controlled trial, with 50 in each group. The study group received TLE using a unique method of "sequential layered application." The control group received 2% lidocaine infiltration anesthesia. Patients rated the pain from the application of anesthesia and from suturing, using a 0-10 visual analog pain scale. 
Follow-up interviews were conducted to assess for complications and to rate patients' wound repair experience. Patients in the study group reported significantly less pain from TLE application, with 66% reporting no pain vs 0% reporting no pain from the infiltration in the control group ( P < 0.001). No difference in pain during wound repair was noted between the 2 groups ( P 0.59). On follow-up, 95% of patients contacted in the TLE group rated their experience in regard to pain as "excellent," compared to 5% of patients in the control group ( P < 0.001). 
Deciphering Drug Concentration
Percentage is measured in grams per 100 mL (ie, 1% is 1 g/100 mL [1000 mg/100 mL] or 10 mg/mL)
Calculate the mg/mL concentration quickly from the percentage by moving the decimal point 1 place to the right, as follows:
Dyclonine 0.5% = 5 mg/mL
Viscous lidocaine 2% = 20 mg/mL
Benzocaine 20% = 200 mg/mL
Adverse effects are usually caused by high plasma concentrations of topical anesthetics that typically result from excessive exposure caused by application to abraded or torn skin. [3, 12] Possible adverse effects include the following:
Burning or stinging may occur local to the administration site.
Oral viscous lidocaine may cause systemic toxicity, particularly with repeated use in infants or children.
CNS: High plasma concentration initially produces CNS stimulation (including seizures), followed by CNS depression (including respiratory arrest). The CNS stimulatory effect may be absent in some patients, particularly when amides (eg, tetracaine) are administered. Solutions that contain epinephrine may add to the CNS stimulatory effect.
Cardiovascular: High plasma levels typically depress the heart and may result in bradycardia, arrhythmias, hypotension, cardiovascular collapse, and cardiac arrest. Local anesthetics that contain epinephrine may cause hypertension, tachycardia, and angina.
Gag-reflex suppression may occur with oral administration.
Other body systems can also experience adverse effects.
Transient burning sensation
Tissue necrosis and sloughing
Methemoglobinemia (with prilocaine)
The United States Food and Drug Administration (FDA) has issued an advisory regarding risk of serious adverse effects with use of topical anesthetics for cosmetic procedures. Life-threatening adverse effects occurred following topical anesthetic application over large surface areas of the body; for some, a plastic occlusive was also applied to enhance absorption. Two women experienced seizures, coma, and death following applying topical anesthetics to their legs with an occlusive dressing before laser hair removal.
For more information, see Life-Threatening Side Effects with the Use of Skin Products Containing Numbing Ingredients for Cosmetic Procedures. This warning was reissued by the FDA in January 2009 following the publication of a study using topical lidocaine to reduce discomfort during mammography.  For more information, see the FDA Public Health Advisory.
Warnings Regarding Cocaine as a Topical Anesthetic
Various anesthetic mixtures that contain cocaine have been used to anesthetize minor skin lacerations, especially on the face or scalp. One such combination that is extemporaneously prepared by hospital pharmacies includes tetracaine 0.5%, epinephrine (Adrenaline) 1:2000, and cocaine 11.8%. This combination is commonly referred to as TAC solution. Its use results in decreased pain on application and may provide better patient tolerance of suturing, particularly in those who are unable to tolerate injections or who have difficulty following instructions or sitting still (eg, children, individuals with mental disabilities). However, serious toxic effects (eg, seizures, cardiac death) have been described following topical cocaine application, particularly in infants and children. [3, 12] Because of increased toxicity, expense, and federal regulatory issues, cocaine is no longer recommended for topical anesthesia.
Compounded mixtures such as lidocaine, epinephrine, tetracaine (LET) solutions, have replaced cocaine with lidocaine 4% because of their superior safety when applied to nonintact skin. Still, these solutions should not be applied to wounds with end-arteriolar blood supply.
Allergic Reaction to Local Anesthetics
Actual hypersensitivity is rare and accounts for less than 1% of all reactions to local anesthetics. Allergic reactions may be attributed to other factors such as acute toxicity, concurrent drug therapy (eg, tachycardia caused by epinephrine), or preservatives, such as paraben or sulfites, that the product may contain. [3, 12]
Local anesthetics with a para -amino benzoic acid (PABA) ester-type structure seem to cause most anesthesia-related allergic reactions. Consequently, use esters (eg, tetracaine, benzocaine) with caution or use a topical anesthetic from the amide class (eg, dibucaine, lidocaine). Documented cross-sensitivity has been exhibited within the ester-based local anesthetics and structurally related compounds (eg, paraben preservatives). Hypersensitivity to the amide local anesthetics is rare. [2, 4] To quickly determine whether an anesthetic agent is an ester or an amide, note whether the generic name contains the letter i once or twice. Esters contain the letter i once in their generic names, whereas the generic names for amides contain the letter i twice.
Onset of action, anesthesia depth, and duration of action are determined by the pKa level, pH level, lipid solubility, protein binding, and vasodilatory effects of the specific local anesthetic. These factors also depend on the area of the skin to which the anesthetic is applied, the vascularity of tissues, the surface area, and anesthesia technique. Duration of application is important to improve topical anesthetic properties. 
|Anes-thetic Class||Generic Name (Trade Name)||
|Dosage Form(s)||Max. Adult Topical Dose, mg||Max. Adult Mucosal Dose, mg||Max. Pediatric Mucosal Dose, mg/kg||Peak Effect, Minutes||Duration, Minutes|
|Amides||Dibucaine (Nupercainal)||1||Cream, ointment||25||--||--||< 5||30-60|
|Lidocaine (Xylocaine, ELA-Max, Lidoderm)||2-5||Viscous jelly, patch, ointment, liposomes||Variable depending on dosage form||250-300||3-4||2-5 Liposome: 30||15-45|
(Americaine, Cetacaine, Dermoplast, Solarcaine)
|0.5-20||Aerosol, cream, gels, lotion, ointment, solution||--||--||--||< 5||15-45|
No longer available in US
|0.5-2||Solution, gel, cream||50||20||0.75||3-8||30-60|
|0.5-1||Aerosol, lozenge||--||50||--||< 10||< 60|
(ProctoFoam, Caladryl Cream for Kids)
|1||Aerosol foam, pledgets, cream, ointment||200||--||--||3-5||--|
|1 g/10 cm2 ‡||--||--||
60: 3 cm depth
120: 5 cm depth
|30-60 after removal|
|Lidocaine 70 mg and tetracaine 70 mg (Synera Patch)||--||
Adults or children:
Apply 1 patch 20-30 min before superficial dermato-
*Use lower concentrations for children or patients who are elderly or debilitated.
† Cocaine is generally not recommended for topical or mucosal application (see warnings above).
‡This dose also applies to the pediatric population (maximum application time in children aged < 3 mo = 1 h; >3 mo = 4 h).