Direct application of henna to the skin to create a temporary tattoo is a process known as mehndi. Henna is a plant-based coloring that is approved in the United States only as a hair dye. It is not approved for direct application to the skin.
The U.S. Food and Drug Administration (FDA) has received reports of skin injury when henna products are used to create temporary tattoos. The most common problem caused by henna is a skin reaction (contact dermatitis) to the pigment in the dye. Permanent loss of skin color (hypopigmentation) in the design of the original tattoo has also been reported.
The risk of developing a severe allergic reaction to henna increases after an episode of contact dermatitis. For that reason, it is recommended that you avoid hair dyes containing henna if you have had a previous problem with an allergic reaction to henna in a temporary tattoo.
Your first henna tattoo should be in a place where you can cover it if it lasts longer than you expect. It is not a good idea try to remove your henna tattoo; let it naturally wear off. Be very careful not damage your skin-do not scrub or pick at your tattoo.