A lot of people are wondering why the OTC failure rate for head lice is so high in many areas. Sometimes it can be because the OTC treatment was not applied correctly, but many times it is because of resistant head lice. It is a controversial subject, however many scientific studies show that persistent head lice are now a reality in many regions and countries around the developed world.

Resistance to lindane for head lice has been highlighted since the 1970s in Britain and the Netherlands. Lindane has been banned in 2009 and although there is an exception for the treatment of head lice, it is not recommended to use this product, as it is highly toxic.



For other pesticides, the first cases of resistant head lice are reported in medical journals from the mid-1990s. In 1999, in some regions of the United Kingdom, in vitro experiments with permethrin and malathion show resistance up to 87% and 64%. According to the journal Archives of Dermatology published by the American Medical Association,  since 2002 many studies agree: the resistance of lice has become a reality and the effectiveness of lice treatements and shampoos is declining.

The lice resistance to permethrin, used in many shampoos and treatments was demonstrated in 1999 by the team of Dr. Pollack in the journal Archives of Pediatric and Adolescent Medicine. The study compared the effectiveness of permethrin head lice treatment products on children of the United States and Borneo. The former had already received treatment containing permethrin, and the latter did not. The results show that among American children previously treated, permethrin did not work. Its effectiveness was close to zero. In the case of children of Borneo not treated before, the mortality of lice increased with the concentration of permethrin.

A study has been conducted in Florida in 2004 by the Departments of Dermatology and Medicine, University of Miami School of Medicine, Florida, and Merck Research Laboratories, West Point, Pennsylvania. It was designed to determine if Nix 1% Permethrin Crème Rinse Lice Treatment (1% PLT) would effectively treat ≥95% of patients on day 2 or on day 15. The study was conducted on infested adults and children. All patients were treated with 1% PLT on day 1 and, if still infested, on day 8. The results showed that the lice-free rate was an average of 83.1% on day 2, 45.8% on day 8 (before second treatment), and 78.3% on day 15 (after second treatment). Conclusions: In this population, 1% PLT was significantly less than 95% effective and suggests resistance to 1% PLT.



Malathion 0.5% has been the most prescribed pediculicide in the United Kingdom for around 10 years, and is widely used in Europe and North America. Anecdotal reports suggest malathion treatments are less effective than formerly, but this has not been confirmed clinically. A study was conducted by the Medical Entomology Centre, Insect Research & Development Limited, Royston, United Kingdom, to determine whether malathion is still effective. About only 34% of infested subjects treated with malathion were cured, which means a resistance of about 66%.

If you choose to use a head lice shampoo such as Nix lice treatment or RID head lice treatment, make sure you are not in an area or region with persistent head lice. And never apply such a treatment more than twice if you see no results, as this would be useless and potentially harmful to your kid’s health, as these products can have dangerous side-effects if over-applied.

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