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

Objective: To investigate the safety of over-the-counter malathion-containing head lice preparations by applying a clinical dose to the scalp and measuring the amount of malathion absorbed, any effects on plasma and erythrocyte 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 with one of four malathion-based preparations using the recommended application procedure from the product licenses. Volunteers were randomised to one of six treatment groups comparing the application of aqueous-based versus alcohol-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 malathion metabolite excretion. Hair washings were collected after each application to assess malathion content. Results: A typical dose of head lice preparation delivers 0.1 to 0.2g of malathion to the scalp, of which one-third to two-thirds is washed off the hair following the allocated treatment time. No clinical effect was noted on plasma or erythrocyte cholinesterase levels irrespective of application conditions. Approximately 0.2 to 3.2% of applied malathion was eliminated in the urine as metabolites with levels decreasing to baseline values by 96 hours. Conclusion: This study found no evidence of a clinical effect on either plasma or erythrocyte cholinesterase activity with malathion-containing head lice preparations when they were applied, in accordance with the instructions for use, to the heads of healthy volunteers.