Head lice. Dimeticone is the pediculicide of choice.

Infestation of the scalp by head lice, or pediculosis, is a common, unpleasant but harmless parasitosis. For patients with pediculosis, which topical treatment eradicates the parasites effectively while causing the least harm? We reviewed the available evidence using the standard Prescrire methodology. Lice can be eradicated by shaving the head or combing the hair several times a day for several weeks with a fine-toothed lice comb, although combing is only completely effective in about 50% of cases. Pyrethroids (permethrin, phenothrin and bioallethrin), often combined with piperonyl butoxide, are insecticides that are neurotoxic to lice. The lice eradication rates achieved in trials of these agents are highly variable, ranging from 13% to 75% depending on the country, probably due to the development of resistance. In five randomised trials, the organophosphorus insecticide malathion was more effective than permethrin or phenothrin, achieving eradication rates of 80% to 98%. Topical application of the insecticides ivermectin or spinosad was effective in 75% to 85% of patients in randomised trials. Insecticides have mainly local adverse effects: pruritus and irritation of the scalp. Cases of malathion poisoning have been reported following topical application or ingestion. The long-term toxicity of insecticides is unclear; it therefore appears preferable to minimise their use. Agents that kill lice through physical mechanisms have few known adverse effects. It seems unlikely that lice will develop resistance to them. Dimeticone, a silicone compound, is not absorbed through the skin and provokes very few adverse effects. It is one of the better evaluated agents: in three randomised trials, 70% to 97% of patients were lice-free after two weeks. Other agents with a physical action on lice have been evaluated, each in one randomised trial including a few dozen patients. One of these, 1,2-octanediol, applied in an alcoholic solution, seemed to eradicate lice effectively with no notable adverse effects. It is advisable to avoid aerosol formulations due to the risk of bronchospasm, products containing terpenes as these compounds can cause seizures in infants and young children, and products that lack a child-proof cap. In practice, as of early 2014, pyrethroids are no longer the first-choice treatment for head lice: they are losing effectiveness and may be toxic in the long-term. Dimeticone is a better choice, because it has few known adverse effects and proven efficacy.