The pure form of Henna doesn’t have side effects
The purest form of henna, the one that doesn’t include any additives or additional chemicals, is not known to cause any side effects. This type of henna gives a reddish-orange color.
Black Henna is not pure
The pure Henna gives a reddish orange color. Black Henna is obtained after mixing pure Henna with some ingredients, usually Para-phenylenediamine (PPD). In many cases, those ingredients aren’t natural and so they can cause problems.
PPD might cause allergies
Para-phenylenediamine (PPD) is a chemical that is sometimes added to henna to intensify the color. PPD can cause allergic reactions to some people if it came in contact with the skin. This includes itching, redness and burning.
If harmful chemicals are added to the Henna then they might lead to roughness of hair in addition to damaging of hair shafts. Pure Henna doesn’t cause this problem.
If Henna came in contact with the eyes it can cause eye redness.
Lawsone is a mutagen
The dye responsible for Henna’s color is a mutagen and it can cause damage to cell DNA. (See Why do we develop grey or white hair?)
Some people report having dry hair after using Henna. Using a conditioner afterwards can remedy the problem.
Connections with cancer
Some people suggest that the DNA changes Lawsone can cause can lead to cancer. Lawsone is a natural component of Henna and is present in the leaves of its plant.
PPD can cause death
In some rare cases, the unnatural component of Henna, PPD, can lead to death. This is a very rare complication but it can happen with people who are extremely allergic to it.
PPD might be bad during pregnancy
During pregnancy, the immune system gets weaker, and as a result, the PPD in black henna might cause allergic reactions. (See Why Keratin hair treatment is bad for you?)