We found two systematic reviews.2,3 The first (search date March 1995, seven randomised controlled trials (RCTs), 1808 people) of 11 insecticide products included lindane, carbaryl, malathion, permethrin and other pyrethroids in various vehicles.3 Two RCTs were identified as showing that only permethrin produced clinically significant differences in the rate of treatment success; both compared lindane (1% shampoo) with permethrin (1% crème rinse).4,5 Permethrin was found to be more effective (lindane versus permethrin; odds ratio (OR) for not clearing head lice 15-2, 95% confidence interval (CI) 8-0-28-8).

The more recent systematic review (search date May 1998, updated February 2001) set stricter criteria for RCTs and rejected all but four trials.2 It excluded both studies on which the earlier review was based. One RCT (63 people) looked at the effect of permethrin (1% crème rinse) compared with the base formulation minus the permethrin (placebo). It found that, after 7 days, permethrin was more effective against head lice than a placebo (29/29 people had no lice with permethrin compared with 3/34 with placebo;

relative risk (RR) 11-3, CI 3-9-33-4; number needed to treat (NNT) 1, CI 0-03-0-3). Two weeks after treatment, fewer people who had received permethrin were infected with head lice compared with the placebo group (1/29 (4%) versus 22/24 (92%); RR 0-04, CI 0-006-0-3; NNT

1, CI 4-172).6 One RCT (115 people) compared malathion (0-5% alcoholic lotion) with the vehicle base as placebo. At 1 week, fewer people treated with malathion were found to have head lice compared with the placebo group (3/65 (5%) versus 26/47 (55%); RR 0-08, CI 0-03-0-3; NNT

2, CI 4-39).7 One quasi-randomised study (22 people) comparing synergised pyrethrin (0-16% mousse) with permethrin (1% crème rinse) found that, at 6 days, people treated with pyrethrin had no lice compared with permethrin (17/19 versus 3/3; RR 0-89, CI 0-8-1-0; NNT 10, CI 1-1-3).8

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