GHK Cu is one of my personal favorites for a lot of reasons, but I can say without a doubt that it's use effectively stopped and reversed, my receiving hairline. However, there are Selected Biomarkers Revealed Potential Skin Toxicity Caused by Certain Copper Compounds Scientific Reports GHK Cu for Skin & Wound Healing: Current Evidence Meeting Point Health GHK Cu as a multifunctional copper peptide: synthesis routes, process engineering and emerging applications Systems Microbiology and Biomanufacturing Springer Nature Link