¹ Fluhr, J. W., Vienne, M.-P. ., Lauze, C., Dupuy, P., Gehring, W., & Gloor, M. (1999). Tolerance Profile of Retinol, Retinaldehyde and Retinoic Acid under Maximized and Long-Term Clinical Conditions. Dermatology, 199(1), 57–60 

² Saurat, J. H., Didierjean, L., Masgrau, E., Piletta, P. A., Jaconi, S., Chatellard-Gruaz, D., Gumowski, D., Masouyé, I., Salomon, D., & Siegenthaler, G. (1994). Topical Retinaldehyde on Human Skin: Biologic Effects and Tolerance. Journal of Investigative Dermatology, 103(6), 770–774. 

³ Siegenthaler, G., Jean‐Hilaire Saurat, & Ponec, M. (1990). Retinol and retinal metabolism. Relationship to the state of differentiation of cultured human keratinocytes. Biochemical Journal, 268(2), 371–378. 

⁴ Brown, A., Furmanczyk, M., Ramos, D., Adrià Ribes, Pons, L., Bustos, J., Fernández, A. R., Granger, C., & Jourdan, E. (2023). Natural Retinol Analogs Potentiate the Effects of Retinal on Aged and Photodamaged Skin: Results from In Vitro to Clinical Studies. Dermatology and Therapy, 13(10), 2299–2317. 

⁶ Clinical testing via visual assessment using Griffith’s 10-point scale analysis on 28 participants immediately after the first application.

⁷ Clinical testing via expert evaluation of 28 participants immediately after the first application.

⁹ Clinical testing via expert evaluation of 27 participants immediately after the first application.

¹⁰ Clinical testing with 28 participants after twice a day use for 28 days.

¹³ Clinical testing with 28 participants based on the best result after twice a day use for 28 days.

¹⁵ Expert grading with 27 participants after twice a day use for 56 days.

¹⁶ Clinical testing with 27 participants after twice a day use for 56 days.