The US regulator, the Food and Drug Authority, has warned that black mehndi is not an approved substance for use on human skin.

Mehndi in its pure form comes from a plant and is only approved in the US for hair dye. This pure form of henna looks reddish brown, or orange or brown. If someone is selling ‘black mehndi’ it means that they are adding some colour to make it black, because that is not the natural shade, the FDA has warned.

“Even brown shades of products marketed as henna may contain other ingredients intended to make them darker or make the stain last longer on the skin,” the FDA said.

“The extra ingredient used to blacken henna is often a coal-tar hair dye containing p-phenylenediamine (PPD), an ingredient that can cause dangerous skin reactions in some people.”

This is why a patch test on a small spot of skin is always recommended for hair dye so that the person using it does not have a reaction.