Don’t eat henna; it’s toxic.
The main ingredient in henna dye is hydroxynaphthaquinone which is obtained from leaves of the Lawsonia tree. Hydroxynaphthaquinone alone is not carcinogenic, but when combined with para-phenylenediamine (PPD) the mixture has been documented as causing problems. PPD is a synthetic dye used to accelerate the drying time of the henna mixture, darkens the overall tattoo and allows for more precise, it is also known as black mehndi. The chemical has been documented to cause severe blisters, skin rashes and scars.
In 1979 the US National Cancer Institute labeled PPD as a possible carcinogen due to a high incidence of liver cancer in workers at PPD production plants. Not all henna artists or henna kits use dye with PPD, if the resulting tattoo is an intense dark black and lasts for more than several weeks, it likely contains the undesired ingredient. In other words, check if the henna contains this ingredient, and if it doesn’t you are good to go!
If you want to read more dangers of henna then click here.