AbstractObjectiveSmall molecules, like panthenol have been shown to penetrate hair. However, their interactions with the protein structures of hair have not been well studied. In this study, the penetration of panthenol into protein structures of hair was confirmed, and NMR methods were utilized to understand the protein–panthenol interaction. This interaction is likely the source of the tensile strength benefits measured with panthenol for hair.MethodsCross‐sections of hair treated with deuterium‐labelled panthenol were studied using NanoSIMS to visualize the location of panthenol within hair. Solid‐state NMR (both 1H–13C and 1H–2H) was employed to measure panthenol interactions inside hair using deuterium‐labelled panthenol. Stress/strain curves—where stress is defined as average force per unit area—were measured for hair treated with and without panthenol to generate tensile strength data.ResultsPenetration of panthenol into hair was demonstrated, with images indicating specific penetration into cortical protein regions. NMR results from various experiments provided evidence of direct interaction between the N–H of panthenol and protein aromatic side chains. A ‘shared’ proton was identified between panthenol and hair proteins, indicating spatial proximity and interaction. Break stress and elastic modulus of hair treated with panthenol were higher than those of control hair, suggesting that bonds—likely hydrogen bonds—were formed between panthenol and hair.ConclusionPanthenol can deliver tensile strength benefits to hair. This benefit is proposed to result from hydrogen bonds formed between panthenol and hair proteins.