Validation of diphenhydramine as a dermal local anesthetic

Although diphenhydramine has been recommended as an alternate local anesthetic for patients claiming allergy to lidocaine, no prior placebo-controlled evaluations of diphenhydramine for dermal anesthesia have been performed. We sought to determine the relative efficacy of diphenhydramine compared to saline placebo and lidocaine.

Prospective, randomized, double-blind, placebo-controlled clinical trial.

Twenty-four healthy adult volunteers.

Subjects received intradermal 0.5-mL injections of diphenhydramine 1%, diphenhydramine 2%, lidocaine 1%, and normal saline placebo in a randomized, double-blind fashion. Extent of anesthesia (in mm2) was assessed at one, two, five, ten, 20, and 30 minutes. Pain of initial infiltration was assessed with a visual analog scale.

Diphenhydramine 1% produced greater anesthesia than placebo (P < .001) and equivalent anesthesia to lidocaine 1% (P = .889). (Our sample size had 90% power to detect a difference of 30% from the peak anesthesia observed.) Diphenhydramine 2% was less effective than diphenhydramine 1%; however, this difference was not statistically significant (P = .295). Infiltration of either concentration of diphenhydramine was significantly more painful than either lidocaine or saline (P < or = .001) for all comparisons). No clinically important complications were noted.

In this study of 24 adult volunteers, diphenhydramine 1% was as effective as lidocaine 1% for achieving dermal local anesthesia, although injection was more painful. Although no clinically important complications were noted in our study, the safety of diphenhydramine remains to be established, especially in areas with poor collateral perfusion (eg, digits, pinna, and nose).