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This article is for Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Head Lice and Nits article more useful, or one of our other health articles.

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See separate Pubic and Body Lice article.

Head lice infestation (pediculosis capitis) is caused by the parasitic insect Pediculus humanus capitis, which lives on and among the hair of the scalp and neck of humans. The adult louse feeds on blood.

Male and female head lice size comparison

Head lice with match/comb
Pediculosis, CC BY-SA 3.0, via Wikimedia Commons

Image via Wikimedia Commons

  • Louse eggs (ova or nits) are small, oval and yellowish white and are attached to the hair shafts. They usually take 7-10 days to hatch.
  • Immature lice (nymphs) take 7-10 days to mature into adults.
  • Adult lice are up to 3 mm in length, said to be approximately the size of a sesame seed. They have six legs (which have hook-like claws to hold on to the hair) and are grey-white to black in colour. They do not have wings and cannot jump or fly. The female louse lays up to eight eggs per day.
  • Adult lice survive by taking a feed of blood from their host several times a day. They can live for about 30 days.
  • Transmission of head lice usually requires head-to-head contact. Head lice can only live on humans and the lifespan is very short (several days) once detached from a human head.
  • Head lice infestation is extremely common worldwide, affecting millions of children. Prevalence varies between countries.
  • Head lice are endemic in the UK. They are most common in children aged 4-11 with a peak age of 7-8 years. Information on current prevalence in the UK is limited, but past studies have shown prevalence rates of 4.1-22%.
  • There is no evidence that head lice have a preference for either clean or dirty hair.

Risk factors include:

  • Age less than 12 years.
  • Female gender.
  • Families with four or more children.
  • Lower socio-economic status.
  • Long hair.
  • Many infestations are totally asymptomatic. Presentation is usually when adult lice or nits have been seen. Others present with itching.
  • Itching of the scalp is not sufficient to diagnose active infestation. Itching may not develop for several weeks or months after becoming infested and may persist for days to weeks after successful eradication of head lice.
  • Nits alone are not sufficient to diagnose active head lice infestation because it is difficult to distinguish between dead and live eggs with the naked eye.

A diagnosis of active head lice infestation can only be made if a live head louse is found. Detection combing (systematic combing of wet or dry hair with a detection comb) should be used to confirm the presence of lice:

  • A fine-toothed (teeth 0.2-0.3 mm apart) detection comb should be used. This is different to a nit removal comb, which has narrower gaps. Some nit combs can be prescribed on an FP10; the Bug Buster® comb is the only one of these which has been assessed through clinical trials.
  • Wet combing takes 10-15 minutes per head; lice are immobilised by hair conditioner, so are easy to see on the comb.
  • Dry combing takes at least 3-5 minutes per head. Using a comb on dry hair may produce static electricity; when a louse is spotted on the comb, placing a thumb on it before drawing the comb out of the hair prevents the louse being flicked off the comb by the static electricity in the hair.
  • Itching may also be due to:
    • Psychogenic itch on hearing that there are head lice within the school.
    • Other itchy scalp conditions, such as eczema.
    • Successfully treated head lice infestation but with persisting itch, which may last for weeks.
  • Nits can be confused with:
    • Seborrhoeic scales.
    • Hair muffs (secretions from the hair follicle that are wrapped round the hair shaft).
    • Hairspray.

Nits can usually be differentiated, as they stick firmly to the hair even after vigorous brushing.

General advice:

  • Treat the person only if a live head louse is found. Treat all affected household members simultaneously.
  • Following treatment, success or otherwise should be confirmed (see section 'Confirming treatment success', below).
  • There is no need to wash clothing or bedding that has been in contact with lice.
  • Children who are being treated for head lice can still attend school.[5]
Treatment options are:
  • Use of mechanical methods, ie wet combing.
  • Use of physical insecticides, which act by coating the lice and blocking their oxygen supply. Available products include dimeticone 4% lotion (Hedrin®), dimeticone 92% spray (NYDA®), and isopropyl myristate and cyclomethicone solution (Full Marks Solution®).
  • Use of chemical or traditional insecticides, which act by exerting a neurotoxic action and poisoning the lice. In the UK, the only current one which is recommended is malathion 0.5% aqueous liquid (Derbac-M®). Resistance has developed to previously used insecticides (such as permethrin), which are now no longer recommended. Ivermectin, a chemical treatment, has been used in oral form as an antihelmintic but is being evaluated as a 0.5% solution for the topical treatment of head lice resistant to other treatment[6, 7].

Choice of treatment depends largely on individual preference. Wet combing or dimeticone 4% lotion are first-line choices for pregnant or breast-feeding women, children aged 6 months to 2 years, and people with asthma or eczema. All insecticides should be applied twice, at least seven days apart, in order to treat any lice hatching from eggs before they lay more eggs themselves.

Dimeticone 4% lotion (Hedrin®)

  • This should be applied twice, with seven days between applications.
  • Dimeticone is applied to dry hair, then left on the hair and scalp for eight hours, or overnight before being washed out using shampoo.
  • One 50 ml bottle is enough for short or shoulder-length hair but the larger bottle (150 ml) is needed for longer hair.
  • Other formulations are available to buy over the counter.
  • It is odourless and well tolerated but is not very effective against the eggs.
  • Studies report cure rates of 70% in the UK. Some studies have reported success after just one application[8].

Dimeticone 92% spray (NYDA® spray)

  • This should be applied twice, with seven days between applications.
  • It should be applied to dry hair and left for 30 minutes, after which the hair should be combed through with the comb provided to remove the lice. It is then left on for eight hours, before being washed off
  • One 50 ml bottle is enough for short or shoulder-length hair but for longer hair the larger pack is needed, containing two 50 ml bottles.
  • It is thought to be more effective than dimeticone 4%, with 97% cure rates reported, and has better ovicidal activity.
  • It cannot be used in pregnancy, in women who are breast-feeding, or in children under the age of 2.

Isopropyl myristate and cyclomethicone (Full Marks Solution®)

  • This should be applied twice, with seven days between applications.
  • It is left in place for 10 minutes and the hair is then systematically combed with a fine-toothed comb to remove lice and then washed using a non-conditioning shampoo to remove the solution.
  • Cure rates of 52-82% have been reported in studies.
  • This is not suitable for children younger than 2 years of age, for pregnant or breast-feeding women, or for people with skin conditions. It may be used by people with asthma.

Malathion 0.5% aqueous liquid

  • This is a traditional chemical treatment.
  • It should be applied twice, with seven days between applications.
  • It should be applied to dry hair from the roots to the tips, left on the hair and scalp for 12 hours or overnight and then washed out using shampoo.
  • It has an unpleasant smell and may cause skin irritation in some people.
  • The 50 ml bottle is sufficient for short or shoulder-length hair but for longer hair the 200 ml bottle will be needed.
  • It may be used in pregnant women if wet combing and dimeticone 4% have been ineffective. It may be used at all ages and in those with asthma or skin conditions.
  • Malathion remains a recommended treatment but is there is evidence that (unlike other countries) it is beginning to lose its effectiveness in the UK[9].

Wet combing

  • Advise using the Bug Buster® comb and method, as this has been evaluated in clinical trials. Further information can be obtained from Community Hygeine Concern Bug Busting[10].
  • It involves methodically combing wet hair with the fine-toothed Bug Buster® comb to remove lice. This is undertaken for four sessions over two weeks, on Day 1, 5, 9 and 13.
  • The hair should be washed in the normal way using an ordinary shampoo. After rising, a large amount of conditioner should be applied.
  • A normal comb should be used to remove tangles followed by use of a detection comb.
  • After the whole head has been combed, it should be rinsed again.
  • Wet combing should be continued until no full-grown lice have been seen for three consecutive sessions.
  • 50-60% cure rates have been reported. Generally wet combing is thought to be less effective than insecticides.
  • There are no contra-indications or safety precautions but it is time-consuming.
  • Other mechanical devices such as electronic combs are not recommended, as there is no evidence of their efficacy and there may be safety concerns.
  • Insecticide treatment:
    • Advise people to check whether treatment was successful by detection combing on Day 2 or Day 3 after completing a course of treatment and again after an interval of seven days. Treatment has been successful if no lice are found at both sessions.
    • Finding nits does not constitute treatment failure; only the finding of live lice does this.
    • If treatment has been repeated because the first treatment course was unsuccessful: advise people to use detection combing five days after the first application of treatment, two days after the second application of treatment, and again after a further seven days.
  • Wet combing using the Bug Buster® comb and method:
    • This method has been successful if no lice are found on Day 17. If lice are found on Day 17, continue use of the Bug Buster® comb until no full-grown lice have been found on three consecutive sessions.
  • Household members, close family and close friends (both adults and children) should be assessed using detection combing to identify possible sources of re-infestation.
  • Check whether the treatment was used correctly and that if it was an insecticide it was repeated after seven days..
  • Repeat the same treatment or switch to a different treatment: the choice will depend on the preference of the person or child's parent and whether resistance to a traditional insecticide is suspected.
  • Ensure that all affected household contacts are again treated simultaneously.
  • Advice should be sought from local laboratories if resistance appears to be a problem. The policy of rotating treatments over a complete district is no longer used[3].
  • Pruritic rash on the back of the neck and behind the ears, caused by a hypersensitivity reaction to louse faeces.
  • Excoriation, skin infection and impetigo may occasionally occur.
  • Loss of sleep caused by continuous itching is occasionally a problem.
  • Anxiety and distress for children and parents.

If left untreated, infestation with head lice may persist for long periods, often for more than one year.

  • It is very difficult to control the spread of head lice in children, due to the close contact that children normally have with each other.
  • There is no evidence for any benefit of head lice repellents, or using head lice treatments prophylactically.
  • There is no evidence that measures beyond normal personal hygiene, housekeeping and laundry can prevent re-infestation.
  • If a head lice infestation is noted in a school, vigilance amongst the parents and treatment of affected children will help to prevent a cycle of re-infestation. The school nurse can play an effective role in teaching families the correct method of detection combing. Alert letters from schools are not recommended, however, as there will probably always be a child with nits in the school and it may cause alarm and unnecessary treatments.
  • Tying back long hair and regular (every 3-4 days) methodical combing with a fine-toothed comb may help to prevent infestation.

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Further reading and references

  • Burgess IF; Head lice. BMJ Clin Evid. 2011 May 162011. pii: 1703.

  1. Head lice; NICE CKS, February 2015 (UK access only)

  2. Head lice (Pediculosis); Public Health England

  3. Head lice: Evidence-based Guidelines based on the Stafford Report; Public Health Medicine Environmental Group, 2012

  4. Head Lice: Questions and Answers for Healthcare Professionals; NHS Wales January 2014

  5. Guidance on infection control in schools and other childcare settings; UK Health Security Agency (September 2017 - last updated April 2022)

  6. Pariser DM, Meinking TL, Bell M, et al; Topical 0.5% ivermectin lotion for treatment of head lice. N Engl J Med. 2012 Nov367(18):1687-93. doi: 10.1056/NEJMoa1200107.

  7. Smith CH, Goldman RD; An incurable itch: head lice. Can Fam Physician. 2012 Aug58(8):839-41.

  8. Burgess IF, Brunton ER, Burgess NA; Single application of 4% dimeticone liquid gel versus two applications of 1% permethrin creme rinse for treatment of head louse infestation: a randomised controlled trial. BMC Dermatol. 2013 Apr 113:5. doi: 10.1186/1471-5945-13-5.

  9. Durand R, Bouvresse S, Berdjane Z, et al; Insecticide resistance in head lice: clinical, parasitological and genetic aspects. Clin Microbiol Infect. 2012 Apr18(4):338-44. doi: 10.1111/j.1469-0691.2012.03806.x.

  10. Bug Busting; Community Hygiene Concern, 2013