research Apply directly to the forehead: Holmes, Nana, and hennapecia March 2007 in “The FASEB Journal” Henna mixed with PPD can cause skin reactions, scarring, and a specific type of baldness, and needs more research to understand these effects.
research Scanning Electron Microscopy Approach for Evaluation of Hair Dyed with Lawsonia inermis Powder: in vitro Study 8 citations , November 2019 in “International Journal of Morphology” Henna dye improves hair cuticle and thickness but effects vary by hair type and health.