N,N-diethyl-m-toluamide (DEET) induces favorable repellency against insects by acting on the sensory nervous system. According to emerging literature reports, DEET side effects in humans involve new molecular targets including the cholinergic system, acetylcholinesterase (AChE), muscarinic M1 and M3 receptor and the participation of the second messenger nitric oxide (NO). Most of these molecular events targeted by DEET have previously been characterized in insects while they have been considered as marginal compared to classical repellent properties. Despite these uncommon actions in insects, there is no consensus on the effects in human. Based on these data, this review provides new insights on side effects in human and more largely in mammals by identifying the unusual properties of DEET in insects, which seem to be correlated with adverse effects in mammals. These data will be very helpful to understand better the toxicological effects observed in order to protect non-target organisms from the toxicity.