Permethrin (Elimite, Acticin, Nix Creme Rinse) belongs to a class of drugs called pyrethrins and is used to treat head lice (Pediculus humanus var capitis) and crab lice (Pthirus pubis) and their nits or eggs, and scabies caused by a mite infestation (Sarcoptes scabiei). Pyrethrin is a derivative of the Chrysanthemum flower. It is usually combined or synergized with piperonyl butoxide. Together the two chemicals act synergistically to kill lice, by acting on nerve cell membranes and interrupting signals travelling between the brain and the muscles. The lice/parasites become paralyzed and die because they are unable to breath. Pyrethrin also has some effect on the nits or eggs of the lice. Permethrin is available as a cream and is applied to the skin, or as a lotion applied to the scalp. Permethrin 1% (Nix) is available over-the-counter, and permethrin 5% (Elimite) is available by prescription. The most common side effects reported with permethrin are itching; mild burning or stinging; redness; and swelling. Severe reactions include rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue. If you experience any of these symptoms, call your physician immediately.