Demecarium (HUMORSOL®) is an indirect-acting parasympathomimetic agent, also known as a cholinesterase inhibitor and anticholinesterase. Cholinesterase inhibitors prolong the effect of acetylcholine, which is released at the neuroeffector junction of parasympathetic postganglion nerves, by inactivating the cholinesterases that break it down. Application of demecarium (HUMORSOL®) to the eye produces intense miosis and ciliary muscle contraction due to inhibition of cholinesterase, allowing acetylcholine to accumulate at sites of cholinergic transmission. These effects are accompanied by increased capillary permeability of the ciliary body and iris, increased permeability of the blood-aqueous barrier, and vasodilation. Myopia may be induced or, if present, may be augmented by the increased refractive power of the lens that results from the accommodative effect of the drug. Demecarium (HUMORSOL®) indirectly produces some of the muscarinic and nicotinic effects of acetylcholine as quantities of the latter accumulate.