No, henna isn’t bad for your hair, and for many people provides strengthening and conditioning properties. Many cosmetologists think it’s bad because they don’t know much about it and don’t know that there’s a difference between pure henna (BAQ=Body Art Quality) and henna that’s been mixed with other things that may be bad for hair. The henna itself isn’t the problem in that case, but rather the other chemicals that have been added.
Some people will experience some dryness after using henna, but that is easily corrected with a moisture treatment, and it doesn’t last. Overall, henna is MUCH better for hair than commercial dyes so just make sure you buy good body art quality.
Also stay away from ‘black henna’ or any henna products that promise to turn your hair black (except for indigo-based products, which are safe and often mistakenly labelled as black henna). Real henna only stains hair and skin orange, red, burgundy, brown and like colours – never black. There’s a good chance that any black henna products you buy have PPD (Para-phenylendiamine) in them. And even if they don’t, you know that some unnatural ingredients were added in order to alter the henna’s natural colour, so it’s best to steer completely clear of these items.