Butyrylcholinesterase (BChE), often referred to as pseudocholinesterase or cholinesterase, is a B-esterase synthesized in the liver and secreted in the plasma, whose physiological role is still unclear, though it hydrolyzes a variety of choline esters and many drugs, including drugs of abuse. OPs can bind to BChE and inhibit its activity. High levels of BChE are present in plasma; by scavenging OPs, BChE can guard against their toxicity, as the OP would be unavailable for reaction with the primary target AChE [44]. A large number of genetic polymorphisms have been described for BChE. In addition to the wild-type allele, there are at least 39 identified genetic variants with nucleotide alterations in the coding region [52]. Several of the these variants are silent (i.e., they have 0–10% of normal activity), but they are rare; most common variants (e.g., atypical variants) have a reduced activity and are far less efficient scavengers of positively charged cholinesterase inhibitors [52]. Individuals with genetic variants of BChE with no or low activity would be predicted to be more susceptible to OP toxicity. A study in Brazilian farmers supports this hypothesis [53] but additional investigations are certainly warranted.