A randomized, single-dose, parallel group, comparative PK study. Patients will be randomly assigned in 1:1 ratio to the study products. Twenty four (24) consenting patients, age 18 years and older, who meet Inclusion/Exclusion criteria will be enrolled in this study. 12 Patients will be treated with Malathion Gel, 0.5% and other 12 Patients will be treated with Ovide Lotion 0.5%. The study will be single centre and conducted only in India. The primary objective of this study is to compare the systemic exposure of Malathion 0.5% Gel manufactured by Taro Pharmaceuticals USA, Inc. to that of the OVIDE Lotion 0.5%, currently marketed by Taro Pharmaceutical USA, Inc., in adult Patients with head lice. The secondary objective is to determine the link between the exposure and cholinesterase activity.