Not sure. I don't use henna myself.
I read this on tightlycurly though. I like how she has an Ingredients Dictionary.
"Can build up over time and cause hair to become brittle or sticky [Begoun (Hair) pg 399]. Though it's known to be safe, and colorless versions are sold as being conditioning, to increase body, and emphasize highlights, it can cause brittleness in the hair with repeated use. This is because it coats the hair with a thin film of protein-like material. "
But I think the people who use it can truly say whether or not, y'know.
Plus, she does not use Henna.
I thought about henna'ing, but I think I'll have to see what others say about it.
I'll definitely be doing more research (Because it's not good to rely on one source).
I'm subbed to a youtuber who uses henna, and she has has nice results.
And it does not look like she's had adverse effects.