any of several drugs that prevent destruction of the neurotransmitter acetylcholine by the enzyme acetylcholinesterase within the nervous system (nervous system, human). Acetylcholine acts to transmit nerve impulses within the parasympathetic nervous system—i.e., that part of the autonomic nervous system that tends to induce secretion, to contract smooth muscles, and to dilate blood vessels. In preventing the destruction of acetylcholine, anticholinesterase permits high levels of this neurotransmitter to build up at the sites of its action, thus stimulating the parasympathetic nervous system and in turn slowing the heart action, lowering blood pressure, increasing secretion, and inducing contraction of the smooth muscles.

Physostigmine and neostigmine are among the principal anticholinesterases. These drugs have only a few clinical uses, mainly in augmenting gastric and intestinal contractions (in treatment of obstructions of the digestive tract) and augmenting muscular contractions in general (in the treatment of myasthenia gravis). Other anticholinesterases in larger doses, however, are widely used as toxins (toxin) that achieve their effects by causing a continual stimulation of the parasympathetic nervous system. parathion and Malathion are thus highly effective agricultural insecticides (insecticide), while sarin, tabun, and soman are nerve gases designed for use in chemical warfare (chemical weapon) to induce nausea, vomiting, convulsions, and death in humans.