Effectiveness of various formulations of local anesthetics and additives for topical anesthesia – a prospective, randomized, double-blind, placebo-controlled study

BACKGROUND Topical anesthesia is used to control pain associated with many procedures in medicine. Today, the product most commonly applied for topical anesthesia in Germany is EMLA® (lidocaine/prilocaine). However, since prilocaine is a methemoglobin-inducing agent, there are limitations to its use, especially in neonates and infants. The aim of this study was to evaluate the effect of prilocaine and lidocaine as well as propylene glycol, a penetration enhancer, and trometamol, a buffer substance, in anesthetic creams. PATIENTS AND METHODS Twenty-nine healthy adults participated in this study. Standardized creams with eight different compositions were applied and left for 20, 40 or 60 min. After exposure to standardized painful stimuli (blunt/sharp with pressures of 0.2, 0.4 or 0.8 N), subjects rated the experimental pain using a visual analog scale. RESULTS Significant results were only found with an exposure time of 60 min and a stamp pressure of 0.8 N. At a concentration of 20%, lidocaine was more effective compared to placebo and equally effective compared to lidocaine/prilocaine in controlling pain. The analgesic effect of the cream containing lidocaine 10% and additional trometamol was significantly superior to that of placebo and non-inferior to that of lidocaine/prilocaine. In this study, the penetration enhancer propylene glycol did not accelerate the onset of the analgesic effect. In contrast, the addition of trometamol (Tris/THAM) accelerated the onset of the effect compared to the native formulation (at 0.4 and 0.8 N). In all of the adult subjects of this study, the minimum exposure time was 60 min for any of the tested topical anesthetic creams. CONCLUSION The results of this study indicate that a cream containing 20% lidocaine, 38% trometamol and 10% propylene glycol may be used as an alternative to lidocaine/prilocaine with a comparable effect and without the need to extend exposure time.