Understanding Permethrin and Its Uses
Permethrin is a synthetic pyrethroid, a medication modeled after naturally occurring insecticides found in chrysanthemum flowers. It is a widely used topical treatment for infestations caused by ectoparasites, primarily scabies (Sarcoptes scabiei) and lice (Pediculus humanus capitis). The medication works by acting as a neurotoxin to these parasites, disrupting their nerve cell membranes, which leads to paralysis and death. It is available over-the-counter (OTC) as a 1% lotion for head lice and as a 5% prescription cream for scabies. Permethrin is considered a first-line treatment due to its high efficacy and safety profile when used correctly.
A Note on Post-Treatment Itching
It is important to know that itching may persist for several weeks even after successful treatment. This is due to an allergic reaction to the dead mites and their feces under the skin and is not necessarily a sign of treatment failure.
How Many Times Can You Be Treated for Scabies?
For a standard case of scabies, the typical treatment involves two applications of 5% permethrin cream. The first application involves applying the cream from the neck down, leaving it on for 8 to 14 hours. A second full-body application is recommended 7 to 14 days later to kill newly hatched mites. Generally, do not use permethrin more than twice without medical advice due to potential skin irritation. Retreatment beyond two doses is only needed if live mites are present 14 days or more after the initial treatment.
Special Case: Crusted (Norwegian) Scabies
Crusted scabies is a severe form requiring more aggressive treatment with both topical and oral medications. Topical permethrin 5% cream may be applied daily or every 2-3 days for one to two weeks, along with oral ivermectin, until the infestation is cured.
How Many Times Can You Be Treated for Lice?
For head lice, 1% permethrin lotion is typically used with two applications. The first involves applying the lotion to damp hair for 10 minutes. A second treatment is usually recommended 7 to 9 days later to kill newly hatched lice. If live lice are still present after the second treatment, an alternative medication may be needed. A three-dose schedule (days 0, 7, and 13-15) has also been suggested for complete eradication.
The Growing Concern of Permethrin Resistance
Concerns about emerging permethrin resistance in scabies and lice are increasing due to reports of treatment failure. However, many apparent failures are due to "pseudo-resistance" from incorrect application, not treating all contacts, or inadequate environmental cleaning. If genuine resistance is suspected after correct application, alternative medications may be prescribed.
Comparison of Permethrin with Alternatives
When permethrin is ineffective or not suitable, alternatives are available, most commonly oral ivermectin for scabies.
| Feature | Permethrin (5% Cream) | Ivermectin (Oral) |
|---|---|---|
| Application | Topical, applied to entire body | Oral tablet |
| Typical Regimen | Two applications, 7-14 days apart | Two doses, 7-14 days apart |
| Efficacy | Considered slightly more effective than single-dose ivermectin; cure rates are comparable after two doses. | Highly effective, especially with two doses. Preferred for large-scale outbreaks or when topical application is difficult. |
| Common Side Effects | Mild, temporary burning, stinging, and itching. | Headache, nausea, dizziness. |
| Use in Pregnancy | Considered safe and is the preferred medication. | Safety not established in pregnant women. |
Other alternatives for scabies include sulfur ointment, benzyl benzoate, and spinosad. For lice, alternatives include malathion, spinosad, and topical ivermectin.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a healthcare professional for diagnosis and treatment.
For more information from an authoritative source, you can visit the CDC page on Scabies Clinical Care.
Conclusion
For standard infestations of scabies and lice, permethrin treatment is typically limited to two applications, spaced about a week apart, to kill both adult parasites and newly hatched larvae. Using permethrin more than twice without a doctor's guidance is not recommended due to the risk of skin irritation. More frequent or prolonged treatment is reserved for severe cases like crusted scabies and is managed by a healthcare professional. True treatment failure may be due to improper application or emerging drug resistance, which would necessitate switching to an alternative medication like oral ivermectin.
