Used to dye women's hair for millennia, henna is making a comeback in health food stores and the shelves of Indian grocers. Unfortunately, the reputation of Lawsonia inermis has been tarnished by misinformation and imposters. The main henna myths are as follows:
Check the ingredient list; if the packet contains anything other than Lawsonia inermis, you do not have pure henna! "Compound henna", a mixture of henna and other herbal or artificial ingredients, has been linked to unfortunate chemical reactions with dyed hair due to the presence of metallic salts. Real henna cannot cause green hair; compound henna can. The best-quality, purest henna for dyeing hair is body-art quality henna, usually sold for mehndi.
Compound hennas are often sold as "Golden brown", "Copper", "Auburn", "Mahogany" and so on – a sure sign that they are not in fact true henna. Real henna comes in exactly one shade – a vivid orange-red. This does not mean it will produce identical results on different heads of hair, however; henna binds to the keratin of the hair and combines its colour with the existing hue. As a result, a platinum blonde using henna is likely to end up with flaming orange hair, whereas a dark brunette may only see a slight reddish tinge in full sunlight. For this reason, "strand testing" your hair before applying the henna is vital.
So-called "black henna" is not in fact henna at all, but either indigo or an artificial compound containing PPD (para-phenylenediamine). The latter is extremely dangerous and can cause hair loss, blistering and scarring, as well as being carcinogenic.
Myth: Henna Will Lighten Hair.
Henna can only darken hair, although the brightness of its orange tone may cause some optical 'highlighting", giving the impression of lighter or brighter hair. Henna also darkens as it oxidises, meaning that the original orange tone visible after the henna is rinsed off may fade over a few days to a deeper red or auburn. Repeated hennaing also darkens hair. For this reason, many people henna their hair repeatedly until it reaches the desired shade, and then only touch up the roots as needed.
Don't make the mistake of thinking that henna, being a natural product, is "gentler" or "weaker" than regular hair dyes in terms of colour permanence! Henna is virtually impossible to strip from hair. Some success may be obtained by repeated oil treatments, honey treatments and/or dyeing or bleaching over the henna, but to fully rid one's hair of the colour is next to impossible. Many people find themselves in the position of having to grow their henna out, or cut it off, in order to get rid of it.
Compound henna and PPD-based "black henna" have given the product a bad name; however, true henna is actually beneficial to hair, as it provides a protein treatment. People with moisture-loving hair may find they need to apply a moisture treatment after hennaing to counteract any brittle or dry feeling; people whose hair loves protein have been known to henna solely for its conditioning benefits.
See also Using Henna to Dye Hair