Fitzpatrick Skin Phototype Is Independently Associated with Differential Short-Term Cutaneous Reactivity Following Standardized Topical Provocation in Humans

Background: Human cutaneous reactivity exhibits marked inter-individual variability, yet the contribution of constitutional pigmentation traits to short-term skin responses remains incompletely characterized. Fitzpatrick skin phototype reflects stable differences in pigmentation-related traits and may therefore act as a phenotypic modifier of early cutaneous reactivity following topical exposure. Methods: In this controlled human study, 239 healthy volunteers were stratified by Fitzpatrick skin phototype into three groups: I–II (n = 138), III (n = 72), and IV–V (n = 29). A standardized emulgel-based topical provocation model was applied under occlusion to the volar forearm, and cutaneous responses were assessed at 20 min (Test A), 24 h (Test B), and 96 h (Test C) using standardized visual scoring. Group comparisons, multivariable linear regression models adjusted for age, sex, country of origin, and experimental lot, and stratified analyses by country of origin, were performed. Results: Early and short-term cutaneous responses differed significantly across phototype groups. Participants with phototypes I–II exhibited higher response scores at both 20 min and 24 h compared with phototype III (p < 0.001). In adjusted models, phototype III remained independently associated with significantly lower reactivity relative to phototypes I–II at 20 min (β = −1.61, p < 0.001) and 24 h (β = −0.98, p < 0.001). Responses among phototypes IV–V were minimal to absent; however, this subgroup was underrepresented, and findings for IV–V are descriptive. Age was a significant positive predictor of response intensity, whereas sex showed no independent association. No persistent reactions were observed at 96 h in any phototype group. Stratified analyses confirmed that the reduced reactivity associated with phototype III was independent of country of origin. Conclusions: Fitzpatrick skin phototype is independently associated with early and short-term cutaneous reactivity following standardized topical provocation in humans. Lighter phototypes (I–II) demonstrate increased susceptibility to transient inflammatory responses, whereas phototype III shows markedly reduced reactivity. These findings support the role of skin phototype as a constitutional modifier of short-term cutaneous responses and highlight the importance of considering pigmentation-related phenotypes in the design and interpretation of dermatological testing, cosmetic tolerability studies, and safety assessments of topical formulations.