Pyrethrin is used to treat head lice (Pediculus humanus var capitis) and crab lice (Pthirus pubis) and their nits or eggs.
Pyrethrin and Permethrin treat lice
It is used to treat scabies caused by a mite infestation (Sarcoptes scabiei). Permethrin 1% cream has been described as “the drug of choice for head lice” (Wolverton, Comprehensive Dermatologic Drug Therapy 2001, 539).
This page describes how pyrethrin and permethrin work and its effectiveness. It also outlines precautions that should be considered when using anti-lice treatments, discusses potential side effects and explains when the treatments should not be used. Finally, the site describes how to use the drugs to eradicate lice.
How do Pyrethrin and Permethrin work?
Both are effective insecticides
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. Synergized pyrethrin has no residual effect meaning that none of the drug remains on the hair following treatment.
Permethrin is a synthetic pyrethroid that is effective against lice, ticks, mites and fleas by acting on the nerve cell membranes interrupting signals travelling between the brain and the muscles. The lice/parasites become paralyzed and die because they are unable to breath. Permethrin has ovicidal activity and can also kill the nits or eggs of the parasites. Permethrin also has residual activity by remaining on the hair following treatment. Permethrin has demonstrated a high cure rate of 97% to 99% in patients who were tested 14 days after a single application.
Studies have shown that synergised pyrethrin and permethrin are of similar effectiveness.
Synergized pyrethrin is available in a 0.33% shampoo or a convenient 2+1 shampoo and conditioner marketed as Pronto Shampoo and R&C Shampoo/Conditioner.
Permethrin is available as a 1% cream rinse (Kwellada-P Creme Rinse or Nix Crème Rinse) for treating head lice and a 5% Kwellada-P lotion or Nix Dermal Cream for treating scabies.
Pyrethrin and permethrin both demonstrate very low dermal absorption and are quickly metabolized to inactive metabolites that are excreted in the urine. Permethrin can be detected on the hair for at least 10 days following application.
How effective are Pyrethrin and Permethrin?
Effective treatments for eradicating head lice
When correctly used, 0.33% synergized pyrethrin and 1% permethrin are usually effective at eliminating head lice. Studies show permethrin to be as or more effective than lindane 1% shampoo (eradication of lice was seen in 97% of patients treated with permethrin and in 67%-85% of those treated with lindane).
It is now highly recommended that all lice treatments be repeated 7-10 days following the first treatment to kill any newly hatched lice. If you suspect that the treatment has not been effective first ensure the product is being applied properly. Synergized pyrethrin is applied to dry hair and permethrin is applied to shampooed and towel dried hair. Lice resistance to permethrin has been reported, but this may be due in part to re-infestation or failure to use the product correctly. (Med Left Drugs Ther 39: 6-7, 1997). Resistance is probably very regional depending on the history of use of the various antiparasite preparations. Studies suggest that treating with a higher concentration of permethrin (5%) or for a longer period of time (overnight) will kill few additional resistant lice. (Arch Pediatr Adolesc Med 1999; 153:969-973)
5% permethrin is not approved by Health Canada for the treatment of head or pubic lice.
For treating pubic lice, most clinicians recommend permethrin 1%, although some recommend permethrin 5% but the evidence is not available to support this practice. As with head lice, you should re-treat the area if lice or eggs are observed 7 days after the initial treatment.
What precautions need to be considered when using pyrethrin or permethrin?
Some standard precautions
These treatments are for external use only. Many people who have scabies or head lice infestation also experience itchiness (pruritus), redness (erythema) and swelling (edema). It is common for pyrethrin or permethrin to exacerbate these conditions.
Those who are allergic to chrysanthemums may experience contact sensitivity to pyrethrin/permethrin, as it is a directly derived or a synthetic derivative of the active insecticidal component of chrysanthemum flowers. Also, since a small amount of formaldehyde is present in permethrin 5%, it is possible that some people may experience contact dermatitis. However, very few reports of sensitivity to these compounds have been confirmed.
Patients with asthma may experience some breathing difficulty or an asthmatic episode.
Pregnant women should use pyrethrin or permethrin only if clearly needed, as no controlled studies have been done to assess potential effects. As it is not known whether the drug is excreted in breast milk, nursing mothers should either stop nursing while using pyrethrin or permethrin or not use the drugs. Patients are cautioned to consult a physician before using lice treatment products in pregnancy, during breast feeding or on children under 2 years of age.
What are the potential side effects?
Side effects may include temporary itching or tingling
The most frequent side effect of permethrin is itching, which is also known as pruritus. This condition is most often a result of the scabies or head lice infestation, and may be heightened after treatment. Oral antihistamines and/or topical corticosteroids can help relieve the itchiness.
When using the 5% cream form of permethrin for scabies, some patients report a mild burning or stinging sensation, mild temporary itchiness, tingling, numbness, mild erythema, edema or rash. The 5% lotion form (for scabies) can lead to similar side effects.
Who should avoid using permethrin?
Those who have hypersensitivity to any synthetic pyrethroid or pyrethrin, to chrysanthemums or to any component of the product, should avoid using pyrethrin or permethrin. If hypersensitivity develops, stop using the product and seek medical advice.
How should Pyrethrin and Permethrin be used?
For external use only
Pyrethrin and permethrin are meant for external use only. Avoid all contact with the mucous membranes (such as the nose and mouth) and avoid contact with the eyes. If pyrethrin or permethrin comes in contact with the eyes, flush immediately with water. If it is ingested, seek medical attention.
Head lice – Administration and dosage
R&C 2+1 Shampoo/Conditioner (0.33% synergized pyrethrin):
Follow directions on the insert. Shampoo/Conditioner lice treatment is applied to dry hair. Saturate the hair and scalp with the product (including the areas behind the ears and on the nape of the neck) and let it remain on the hair for 10 minutes. Rinse hair and scalp thoroughly with water and dry with a clean towel. A fine-toothed comb can be used to remove dead lice and eggs, although it is not required for therapeutic efficacy.
Kwellada-P Crème Rinse or NIX Creme Rinse (1% permethrin):
Follow directions on the insert. Shampoo hair with a non-conditioning shampoo. Rinse well and then thoroughly towel dry hair. Lice treatment conditioner is applied to freshly shampooed and towel dried hair. Saturate the hair and scalp with the product (including the areas behind the ears and on the nape of the neck) and let it remain on the hair for 10 minutes. Rinse hair and scalp thoroughly with water and dry with a clean towel. A fine-toothed comb can be used to remove dead lice and eggs, although it is not required for therapeutic efficacy.
If live lice are observed seven to ten days after treatment, complete a second application of the product.
Scabies – Administration and dosage
For scabies, use 5% permethrin and thoroughly massage the product into the skin from the head to the soles of the feet. Pay particular care to the hands, fingernails and between the digits. If treating infants, massage onto the hairline, neck, scalp, temple and forehead. After 8 to 14 hours (best to leave overnight), remove the cream by washing. A single application of thirty (30) grams / 50 ml, is usually sufficient for an adult. Kwellada-P Lotion (5% permethrin) is to be applied from the neck down. NIX Dermal Cream (5% permethrin) can be used on the whole body including the head.
Choose either Kwellada-P lotion or NIX Dermal Cream depending on your preference for lotion of cream.