If you're like most people, you just ignore that warning and apply the hair dye all over your head. Most of the time it won't be a problem, but you never know which ingredients might irritate your skin and cause a serious reaction.
More: Golden Girls underwear takes granny panties to a new level
Case in point: A Redditor posted a horrifying-looking series of photos that showed a friend's serious allergic reaction to hair dye. Though there's no proof these are real, the succession of photos shows her transition from normal to a very swollen head. The reaction eventually subsides — and her face returns to normal — but she looks like she's in agony.
hair allergic reaction
Comments from other users describe their own reactions to hair dye — and it's not pretty.
"This happened to me. Except my head blistered up as well," one wrote. "I had seeping welts all over my head and down to my nose. There was no skin left on the back of my ears. I would wake in the morning and be stuck to the pillow/sheets. I went insane with the pain. The itchiness of it was so strong that I couldn't physically stop my hands from scratching my head. They had to sedate me, put me on steroids to regrow the skin. Most intense six weeks of my life. I had to use medication to get away from the pain and it's the only time I have had an out of body experience. I will never dye my hair again. The smell of the chemicals makes me itchy."
More: Remarkable makeup artist transforms clients using only her mouth
Another chimed in with a friend story.
"My friend had the same thing happen to her a few years ago... she looked like Sloth from the Goonies. Not one single person said anything, unless she told them what happened first."
Let this serve as a reminder to always do a test first, even if it does delay getting your amazing new color. It'll be worth it.
More: Model's "real selfies" show the behind-the-scenes of being beautiful