There are many medications which can kill lice. Dimethicone is between 70 and 97% effective with a low rate of side effects, and thus is seen as the preferred treatment.[7] It works by physical means and there is no evidence of pesticide resistance.[4] Ivermectin is around 80% effective, but can cause local skin irritation. Malathion has an effectiveness around 90%, but there's the possibility of toxicity.[7] Pyrethroids such as permethrin, while commonly used, have lower rates of effectiveness due to the resistance among lice.[7] Effectiveness varies from 10 to 80%, depending on the population studied.[5][7] Medications within a lotions appear to work better than those within a shampoo.[7]