This study is based on a single-blind, randomized controlled trial. Given potential interindividual variations in skin characteristics may influence skin permeation of topically applied agents (Souto et al., 2022 (link)), a within-subject design was chosen. To our knowledge, only data from studies investigating topical methyl salicylate in combination with other active compounds is available (Higashi et al., 2010 (link); Dölen et al., 2015 (link); Petrofsky et al., 2016 (link); Wang et al., 2022 (link)). A priori sample size calculation for repeated measures ANOVA, within-between interaction (G*power, version 3.1.9.6, Franz Faul, Germany) with an estimated effect size of 0.2, α ≤ .05 and power at 0.8 resulted in a sample size of n = 20. Considering potential dropouts, a total of n = 25 women were recruited from a university population. There was one dropout due to sickness on the day of testing. Three participants had to be excluded due to technical issues (see below), leading to a total sample size of n = 21. The study was approved by the Swiss Ethical Committee of Zurich (KEK-ZH: 2016-01541).
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