Pyrethrin and piperonyl butoxide shampoo is used to treat head lice (Pediculus humanus var capitis) and crab lice (Pthirus pubis) and their nits or eggs in adults and children two years of age and older. It belongs to a class of medications called pediculicides or scabicides. It is available over-the-counter and should only be applied to the skin and the hair. The brand names of products containing pyrethrin and piperonyl butoxide are A-200® Lice Killing Shampoo; Licide®; Pronto® Lice Killing Shampoo; and RID® Lice Killing Shampoo. This medication does have side effects, which may include erythema, pruritus, urticaria, edema and eczema. Pyrethrin and piperonyl butoxide shampoos are FDA pregnancy Category C, which means they are not recommended for use if you’re pregnant because animal reproduction studies have shown an adverse effect on the fetus. However, while there are no adequate and well-controlled studies in humans, the potential benefits may warrant use of the drug in pregnant women despite potential risks.