Henna is a natural product so it generally doesn't have any side effects. It is known to have curative properties and a number of uses. There are however certain instances when people get allergic to henna products. Henna products when not mixed with any chemical or synthetic products, are 100% safe to use and do not have any negative after effects.

Henna Products with Probable Side Effects

Black Henna : There have been instances wherein people using black henna tattoos have complained of skin allergies, rashes and reactions; this is also because of the fact that proper care is not taken after getting the temporary tattoo made. It should also not be used by people who have an aversion to henna.
Henna Hair Dye : Sometimes manufacturers mix synthetic chemicals with henna for better colouring, this can lead to negative reaction.

Natural henna does not have any side effects and is safe to use Henna Mehendi : 100% natural henna will never cause any kind of allergy or skin reaction. Sometimes dye or other chemicals are mixed with henna for better colouring of hands, this results in rashes, red sores in a few cases. It is advisable to have a patch test done first in order to check for any allergic reaction.