Despite the widespread use of henna, reports of contact dermatitis (skin inflammation caused by contact with an allergen or irritant) and other allergic manifestations associated with the coloring have been rare. Henna itself is considered a weak sensitizer. However, people sometimes have allergic reactions to additives that are used to enhance the coloring of henna.

For example, the U.S. Food and Drug Administration (FDA) has warned that “black henna” may contain the “coal tar” color p-phenylenediamine, also known as PPD. This ingredient has been approved for use only as a hair dye, and may cause an allergic reaction when applied to skin. It is used to blacken the coloring of henna, which is naturally reddish-brown.  People who become sensitized to PPD will remain that way for life. This means they will have to exercise caution to avoid exposure to PPD found in some dark-colored cosmetics, black rubber, photocopying and printing inks, ballpoint pen inks, oils, greases and gasoline. Other potential sources of PPD contamination include textile and fur dyes, and photographic developer and lithography plates.

People with sensitivity to PPD must also be careful not to use hair dyes or nail polish that contain the ingredient, and should ask a physician about whether to avoid dark–colored clothing (blue, black or brown) that may contain PPD.

PPD sensitivity may also prohibit the use of substances or medications. These include:

• Sunscreens and creams that contain para-aminobenzoic acid (PABA)
  
• Sulfa medications including some antibiotics
  
• Dyes
  
• Epoxy hardeners
  
• Topical anesthetics such as benzocaine and procaine

The FDA also has warned about the use of other ingredients, such as solvents, used in some henna-based dyes. These are sometimes added to speed up the tattooing process.