Malathion is toxic due to the presence of the carboxy ester group which is readily hydrolyzed by mammalian carboxyesterase in the liver. Both chemicals can be hydrolyzed at the ester linkage, yielding alkyl phosphates and leaving groups, of low toxicity. These are excreted by the body. See Figure 4 below. The mechanism of toxicity of Malathion in insects and humans.
Cholinesterases are the target enzymes for the actions of malathion in the body.
Organophosphates like malathion irreversibly inactivate cholinesterases which are essential in breaking down acetylcholine in nerves, thus prolonging action potentials in nerves, causing spasms, incoordination, convulsions, paralysis, and ultimately death. Many esterase inhibitors potentiate the toxicity of malathion by inhibiting its breakdown. Many of the biproducts of the production of malathion act synergistically to increase its toxicity. Carbamate insecticides, Diazinon, and other insecticides can increase malathion's toxicity as well. Malathion has also been shown to be mutagenic, causing chromosome breaks and aberrations, and sister-chromatid exchanges.