People who have even a single episode of a contact dermatitis or another allergic reaction to henna or another additive associated with mehndi are at greater risk for experiencing a severe allergic reaction with future exposures. For this reason, healthcare professionals urge those with a previous reaction to henna to avoid future mehndi tattoos. In addition, hair dyes that contain henna should be avoided.  

Many people who react to a henna tattoo are allergic to p-phenylenediamine, also known as PPD. This dye, which is added to henna to darken the color, is a potent sensitizer. Those with henna stain allergy must avoid any product that contains PPD. For a listing of products, see Potential causes.

The Fair Labeling and Packaging Act requires that all cosmetics sold on a retail basis in the United States list ingredients on the label. However, this law does not apply to cosmetic samples or products used exclusively by professionals. For this reason, customers must be careful when shopping for or applying cosmetics at a salon or booth at a fair. It can be very difficult to know exactly what is in these products.

Federal law also requires U.S. Food and Drug Administration (FDA) approval of all color additives used in cosmetics (except coal tar used in hair dyes). However, the FDA does not have the authority to approve or reject cosmetic products or ingredients, only to regulate drugs. Cosmetic products that have not been proved safe must carry the following label:

“Warning – The safety of this product has not been determined.”