Insecticides and the skin

Author: Dr Marie Hartley, Staff Writer, 2010.


Insecticides used in dermatology are products designed to kill insects such as lice (head lice, pubic lice, body lice), mites (e.g. scabies), ticks, mosquitoes and fleas. Most of these insecticides are topical preparations (such as creams, lotions, shampoos, foams) or sprays (e.g. permethrin spray). Ivermectin is a systemic medication (tablet) used to treat selected cases of scabies as well as parasitic intestinal worms.

What makes an ideal insecticide?

Commonly used insecticide treatments

Insecticide namePropertiesEffective againstToxicity
e.g. Moov??Head Lice Sensitive
  • Herbal based formulation
  • Includes essential oils
  • May be as or more effective than synthetic insecticides
  • Does not require combing
  • Head lice
  • Non-toxic
Pyrethrum/pyrethrin/phenothrin (can be combined with piperonyl butoxide)
e.g. Full Marks Mousse??(Mousse), Parasidose Extra Strength Lice Shampoo??(Shampoo)
  • Topical agents
  • Naturally occurring insecticide found in the chrysanthemum flower
  • Not ovicidal
  • Head lice
  • Pubic lice
  • Body lice
  • Can be toxic to humans in high doses
  • Seek medical advice before using in children under 2 years of age
Synthetic pyrethroids (e.g. permethrin)
e.g. A-Scabies??(Lotion), Lyderm??(Cream)
  • Topical agents
  • A synthetic form of pyrethrin
  • Effective against adult lice and has some ovicidal activity
  • Head lice
  • Pubic lice
  • Body lice
  • Scabies
  • Can be sprayed onto clothes, tents, and mosquito nets to act as an insect repellent
  • Effective against ticks and fleas
  • Has rarely been associated with toxic effects at high doses, including eye and skin irritation, tremors, loss of coordination, hyperactivity and paralysis
  • Seek medical advice before using in children under 2 years of age
Organophosphates (e.g. malathion, also known as maldison)
e.g. Derbac M (Liquid), A-Lices??Lotion (Lotion), A-Lices??Scalp and Body Hygiene Shampoo (Shampoo)
  • Topical agents
  • Ovicidal
  • Head lice
  • Pubic lice
  • Body lice
  • Scabies
  • Seek medical advice before using in children under 2 years of age
  • Contains flammable alcohol, so do not expose lotion or wet hair to open flame or electric heat, such as a hair dryer
Benzyl benzoate
  • Topical agent
  • Hydrocarbon
  • Seek medical advice before using in children under 2 years of age
Ivermectin Oral tablet
  • Crusted scabies
  • Selected cases of scabies where topical therapy is impractical or has failed
  • Potential neurotoxicity
  • Should not be used in children less than 6 years of age

For insecticides to work properly it is important to meticulously follow the directions, including directions related to product application, laundering of clothes and linen, treatment of contacts and repeat application of treatment. Topical agents 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.

Combing of wet hair using a regular conditioner and a fine-tooth louse comb is the treatment of choice for head lice in children younger than 2 years of age.

Insecticide surface sprays and dusts

A variety of insecticide surface sprays and dusts are available for domestic or commercial control of fleas, bed bugs and other biting insects. These products contain insecticides such as:

Cutaneous adverse effects of insecticides


Permethrin is poisonous to cats. Contact or ingestion may result in skin irritation, poor appetite, vomiting, diarrhoea, tremors and seizures. It may also cause adverse reactions in small dogs (bichon frise, chihuahua, maltese, shih tzu and yorkshire terrier).

Seek medical advice before using in children under 2 years of age particularly in infants under 6 months.

Insecticide resistance

Insecticide resistance is a particular problem in head lice in some regions. There is no information on the resistance patterns of lice in New Zealand. In the USA, head lice resistant to permethrin have been found. In the UK, head lice resistant to both permethrin and malathion have also been detected and are a difficult problem to manage. If a course of treatment fails to cure, a different insecticide should be used for the next course.

Most insecticides currently used to treat head lice are toxic to the parasite's nervous system (neurotoxic). New agents are in development, which work by clogging the respiratory spiracles and asphyxiating the lice. Due to their mode of action, development of resistance is not expected.

Benzyl alcohol lotion is one such agent and has recently been approved by the US Food and Drug Administration as a prescription medication, for use in patients 6 months of age and older. Common side effects of benzyl alcohol lotion include irritation of the skin, scalp, and eyes, and numbness at the site of application. Use in premature infants could lead to serious respiratory, heart- or brain-related adverse events such as seizure, coma, or death.

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