Eserine, also known as physostigmine, is a reversible cholinesterase inhibitor primarily used in the treatment of glaucoma, where it reduces intraocular pressure by increasing the outflow of aqueous humor and inducing miosis (pupil constriction) through elevated acetylcholine levels. It is also employed in the management of myasthenia gravis, a neuromuscular disorder, by enhancing muscle strength via acetylcholinesterase inhibition. Historically, it has served as an antidote for certain neurotoxins affecting the cholinergic system, though this application has diminished with more effective alternatives. Additionally, eserine is utilized in research settings to study the cholinergic system, neurotransmitter mechanisms, and its role in cognitive functions, particularly memory and learning. In ophthalmology, it functions as a diagnostic tool to assess pupil function and the eye's accommodative response.