A phase I volunteer study to establish the degree of absorption and effecton cholinesterase activity of four head lice preparations containing malathion

Objective: To investigate the safety of over-the-counter malathion-containi ng head lice preparations by applying a clinical dose to the scalp and meas uring the amount of malathion absorbed, any effects on plasma and erythrocy te cholinesterase, and the effect of repeat treatment or damaged skin. Design: This was a randomised non-blind study. Study Participants and Interventions: 32 healthy volunteers were treated wi th one of four malathion-based preparations using the recommended applicati on procedure from the product licenses. Volunteers were randomised to one o f six treatment groups comparing the application of aqueous-based versus al cohol-based preparations as: (a) a single dose to intact skin; (b) a single dose to 'damaged' skin; or (c) repeat treatment to intact skin. Main Outcome Measures: Blood samples were collected to allow measurement of plasma and erythrocyte cholinesterase activity. Urinalysis assessed malath ion metabolite excretion. Hair washings were collected after each applicati on to assess malathion content. Results: A typical dose of head lice preparation delivers 0.1 to 0.2g of ma lathion to the scalp, of which one-third to two-thirds is washed off the ha ir following the allocated treatment time. No clinical effect was noted on plasma or erythrocyte cholinesterase levels irrespective of application con ditions. Approximately 0.2 to 3.2% of applied malathion was eliminated in t he urine as metabolites with levels decreasing to baseline values by 96 hou rs. Conclusion: This study found no evidence of a clinical effect on either pla sma or erythrocyte cholinesterase activity with malathion-containing head l ice preparations when they were applied, in accordance with the instruction s for use, to the heads of healthy volunteers.