I agree! Henna takes the curl out of both my hair and my sister's hair. I don't know if one or two treatments would hurt too much, but I was regularly coloring my hair with it and I didn't realize it was flattening my hair until it was too late. I had experienced so many "hair issues" with meds that I never suspected the henna. I went on a Henna Board, and they kept telling me to add more & more henna...and the end result was me literally chopping (with sewing scissors) 6+ inches off my hair.
Then I found this site.
I finally decided that if I'm going to "keep" my red hair, it's going to have to be "unnatural". I use a "demi" hair color, and sometimes just a color rinse, and it covers grays pretty well without disturbing the overall color of my hair. It's just my luck that red hair grays faster and earlier than other colors.