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Permethrin - Indication, Dosage, and Pharmacological Aspects

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Permethrin is a pyrethroid insecticide for lice and scabies, which appears as a yellow-light orange solid or viscous liquid.

Medically reviewed by

Dr. Dhepe Snehal Madhav

Published At April 15, 2024
Reviewed AtApril 18, 2024

Overview:

Permethrin medication treats scabies, which are caused by tiny bugs called mites that make the skin itchy. Permethrin belongs to a group of drugs called Pyrethrins. It works by stopping the mites and their eggs from moving, which kills them. Pyrethrins are natural insect-killing compounds found in chrysanthemum flowers and are often used in household insecticides and products for pets or livestock. Pyrethroids, on the other hand, are man-made chemicals similar to pyrethrins but are usually more toxic to insects and animals and last longer in the environment. While over 1,000 synthetic pyrethroids have been developed, only a few are currently used in the United States, with Permethrin being the most common. Permethrin was approved by the FDA (Food and Drug Administration) in 1998.

Available Doses and Dosage Forms:

Classic Scabies Treatment:

  • For classic scabies, a five percent Permethrin cream is applied all over the body from head to toe and under the fingernails.

  • After eight to 14 hours, the cream is washed off and may need to be reapplied a week later.

  • In severe cases, the cream may be applied daily for a week, followed by twice a week until resolved, along with oral medication.

  • Care should be taken to avoid applying the cream to sensitive areas like mucous membranes.

Head Lice Treatment:

  • To treat head lice, a one percent Permethrin lotion is applied to damp hair and left on for ten minutes before washing it out.

  • A second application may be needed a week later.

  • For pubic lice, the cream is applied and rinsed off after ten minutes, but avoid contact with the eyes.

  • If the cream gets into the eyes accidentally, wash them thoroughly with water.

For Patients:

What Is Scabies and Lice?

  • Lice: Lice are tiny bugs that live on the skin, usually in the hair on the scalp or the pubic area. When someone has lice, it is called pediculosis.

  • Scabies: Scabies is a skin condition caused by tiny mites, which are very small insects that burrow under the skin. Both lice and scabies cause itching.

How Does Permethrin Work?

Permethrin can harm insects if they ingest it or come into contact with it. It works by affecting their nervous system, leading to muscle spasms, paralysis, and eventually death. However, it is more harmful to insects than to humans and dogs because our bodies can break it down faster. Cats are even more sensitive to Permethrin because their bodies take longer to break it down.

What Is the Dosage of Permethrin?

The dosage of Permethrin varies for each person. Follow the doctor's instructions or the directions on the label carefully. The prescribed amount of medicine and how often it has to be used depends on its strength and the specific medical condition it treats.

For Topical Forms (Cream and Lotion):

  • Head Lice: Adults and children aged two years and older can apply once to the hair and scalp. For children under two years, the doctor will determine the appropriate dosage.

  • Scabies: Adults and children aged two months and older can apply once to the skin. For children under two months, the doctor will determine the appropriate dosage.

What Are the Immediate Effects of Brief Permethrin Exposure on Pets and Humans?

Brief exposure to Permethrin can lead to various signs and symptoms depending on how someone comes into contact with it. Pets may show strange behaviors like paw flicking, skin twitching, or excessive drooling. Cats exposed to high levels may seem anxious, have muscle tremors or seizures, and may even die. In humans, skin contact may cause irritation, burning, and itching, while eye exposure can result in redness and pain. Ingesting Permethrin may lead to a sore throat, abdominal pain, nausea, and vomiting. While inhaling, it can cause nose and lung irritation, difficulty breathing, headaches, dizziness, and nausea.

How Effective Is Permethrin?

When Permethrin is applied to the skin, less than one percent of it gets into the body. If someone eats Permethrin, most of it is absorbed quickly. If it is inhaled, it can easily enter the body through the lungs, although specific data on Permethrin inhalation is limited. Once Permethrin is absorbed, it spreads throughout the body rapidly. The highest levels are usually found in the body about three to four hours after ingestion. The body mainly gets rid of Permethrin through urine, but it can also be found in feces. In laboratory tests with rats, about half of the Permethrin was eliminated from their bodies within a day.

What Are the Things to Inform the Doctor Before Taking the Drug?

  • Inform the doctor and pharmacist if an individual has any allergies to Permethrin, Pyrethrins, ragweed, or any other medications or if an individual has had any previous reactions to Permethrin cream or lotion in the past. Ask the pharmacist for a list of the ingredients.

  • Let the doctor and pharmacist know about any other prescription or non-prescription medications, vitamins, supplements, or herbal products they are taking or plan to take.

  • Inform the doctor if a person has any skin conditions or sensitivities.

  • Inform the doctor if a person is pregnant, planning to become pregnant, or breastfeeding. If a person becomes pregnant while using Permethrin, contact the doctor.

How Is Permethrin Administered?

Permethrin Cream:

  • Apply a thin layer of cream all over the body from the neck down to the toes, making sure to get into skin folds and areas like between the toes and fingers, as well as around the waist and buttocks.

  • For babies or adults over 65, apply the cream to the scalp, hairline, temples, and forehead.

  • Leave the cream on for eight to 14 hours, then wash it off by bathing or showering.

  • Even if the skin is itchy afterward, it does not mean the treatment did not work. However, If a person still sees live mites after 14 days, they may need to repeat the treatment.

Permethrin Lotion:

  • Wash the hair with shampoo and towel-dry until damp.

  • Shake the lotion well and apply it to the scalp and hair, making sure to cover everything.

  • Leave the lotion on for ten minutes, then rinse it off with warm water in a sink.

  • Comb the hair to remove any dead lice and eggs (nits).

  • Wash hands and sanitize any clothing, bedding, and personal items that may have come into contact with lice.

What Are the Side Effects of Permethrin?

  • During scabies treatment with Permethrin, some may experience mild itching, burning, or stinging on the skin.

  • Similar sensations may occur during treatment for head lice with Permethrin.

  • A rash could develop as a reaction to Permethrin treatment for head lice.

  • Rarely, tingling, irritation, or allergic reactions may occur with Permethrin use.

  • A very rare case of muscle spasms has been reported, possibly linked to using a stronger concentration of Permethrin.

Overdose:

Permethrin may be harmful if swallowed. If an individual gets overdosed and passes out or has trouble breathing.

Storage:

  • Keep Permethrin medication in its original container, tightly closed, and away from children.

  • Store it at room temperature, avoiding excess heat and moisture (do not keep it in the bathroom).

  • Make sure all medications are stored where children cannot see or reach them, as some containers may not be child-resistant.

  • Always use safety caps and store medications in a secure location.

  • To dispose of unneeded medications safely, do not flush them down the toilet. Instead, use a medicine take-back program.

For Doctors:

Indication:

  • Permethrin, a synthetic chemical, is FDA-approved to treat scabies and head lice.

  • Scabies causes itchy bumps, mainly between fingers, in skin folds, and on the genitals.

  • Permethrin and oral Ivermectin are both effective treatments for scabies, with Permethrin possibly clearing the skin faster.

  • Combining Permethrin with oral Ivermectin is most effective for severe scabies cases.

  • One percent Permethrin lotion is commonly used for head lice, but resistance is growing, prompting research into newer options.

Dose:

The dosage of this medication varies depending on individual patients. Follow the doctor's instructions or the label directions carefully. The prescribed amount of medication and frequency of doses depends on the strength of the medication and the specific medical condition being treated.

For Topical Formulations (Cream and Lotion):

  • Head Lice: Adults and children aged two years and older should apply once to the hair and scalp. Children under two years should have a dosage determined by the doctor.

  • Scabies: Adults and children aged two months and older should apply once to the skin. Children under two months should have a dosage determined by the doctor.

Pharmacological Aspects of Permethrin

  • Pharmacodynamics: Permethrin, which belongs to a group of chemicals called pyrethroids, is effective against a wide variety of pests like lice, ticks, fleas, mites, and other bugs.

  • Mechanism of Action: Synthetic pyrethroids, like Permethrin and fenvalerate, affect the sodium channels in nerve cells. They keep these channels open longer, allowing sodium to enter the cells slowly after they are stimulated. Pyrethroids with an alpha-cyano group, such as fenvalerate, cause a longer-lasting effect compared to other pyrethroids like Permethrin. This prolonged effect may lead to more sensations on the skin.

  • Pharmacokinetics:

  1. Absorption: When an individual puts Permethrin on their skin, only a tiny amount gets into their body, less than two percent.

  2. Distribution: A 2019 study looked at how it attaches to proteins and fats in the blood. They found that most of it sticks to proteins, which helps it travel around. The circulation of Permethrin has not been studied much.

  3. Metabolism: Inside the body, enzymes quickly break down Permethrin, mostly in the liver. This helps make it less harmful.

  4. Excretion: After an individual uses Permethrin, most of it and its byproducts leave the body in urine within three days. It is believed that consuming one to two grams of Permethrin per kilogram of body weight could be fatal.

Toxicity:

  • Studies in animals have shown that Permethrin when given in high doses, can cause lung tumors in mice but not in rats. It has not shown any ability to cause genetic mutations.

  • In humans, consuming one or two grams of Permethrin per kilogram of body weight could be deadly. However, when used topically, there are few reported side effects.

  • Excessive use of five percent Permethrin cream on a child led to signs of toxicity and metabolic acidosis.

  • Two siblings exposed to Permethrin, a type of pesticide, experienced neurological problems. There is no specific treatment for Permethrin poisoning, so doctors focused on supportive care.

Contraindications of Permethrin

  • The FDA has not approved Permethrin for use in infants under two months old, and there have not been many studies on its use in this age group.

  • However, recent research indicates that five percent of Permethrin cream can be safely used to treat scabies in infants.

  • It is important to be cautious when using this medication, as some people may be allergic to it.

  • Permethrin cream should not be used in individuals who are known to be allergic to any of its ingredients or any pyrethroid or Pyrethrin.

Warnings and Precautions:

  • Only for external use only. When a person uses Permethrin for rinsing hair, they should avoid eye contact.

  • If Permethrin has contacted the eyes, rinse immediately.

  • Wash all clothes, bedding, towels, and washcloths in hot water and dry them in a hot dryer for at least 20 minutes. If a person cannot wash something, either dry clean it or seal it in a plastic bag for two weeks.

  • If a person has any wigs or hairpieces, they should shampoo them.

  • Clean the hairbrushes and combs in hot, soapy water for five to ten minutes, and do not share them with others.

  • Vacuum the furniture, rugs, and floors well.

  • Wash all toys, especially stuffed toys used on the bed, in hot, soapy water for five to ten minutes or seal them in a plastic bag for two weeks.

Specific Considerations:

  1. Hepatic Impairment: There is no specific information from the manufacturer about using Permethrin in patients with liver problems.

  2. Renal Impairment: The manufacturer does not mention using Permethrin in patients with kidney issues, but since it is mostly broken down by the liver, it should be safe for these patients.

  3. Pregnancy: Permethrin has not been shown to harm humans during pregnancy and is considered safe for treating conditions like pubic lice and scabies during this time.

  4. Breastfeeding: It is generally okay for breastfeeding mothers to use topical Permethrin because very little of it gets absorbed into the body. However, if mothers are exposed to high levels of Permethrin, such as through certain jobs or malaria control, there could be concerns about long-term effects on the baby. It is recommended to use cream, gel, or liquid products on the breast rather than ointments to minimize the baby's exposure to certain ingredients. A study found that the levels of Permethrin in breast milk were within safe limits.

Dr. Dhepe Snehal Madhav
Dr. Dhepe Snehal Madhav

Venereology

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