The 45-year-old star, who plays forensic scientist Abby Sciuto on the hit show, warned her Twitter followers to be careful about hair dye allergies in order to avoid having their heads "swell up like a melon" or worse.
Perrette is a natural blonde but she has been dyeing her hair for the past 20 years.
"The other half of my face had become twice the size of my head," she told CBS Los Angeles.
She experienced warning signs six months ago when a rash developed on her scalp and neck, but ignored it. Perrette is considering natural hair dye or possibly wearing a wig.