Deltamethrin is a type II pyrethroid insecticide and a modulator of voltage-gated sodium channels (Nav). It binds to Drosophila para Nav channels in a use-dependent manner following brief depolarizing prepulses and with increasing concentrations of 1 to 5 nM, indicating that it preferentially binds to the channel in the open state. It slowly activates Nav1.8 channels, delays inactivation by longer than 40 ms, and induces persistent tail currents for channels expressed in X. laevis oocytes. Deltamethrin decreases proliferation of MCF-7 cells and inhibits androgen receptor transactivation when used at concentrations of 10 and 20 μM or greater, respectively. However, it is not cytotoxic to MCF-7 and CHO cells at concentrations of 25 and 100 μM, respectively, and does not induce transactivation of the estrogen receptor.