An acetylcholinesterase inhibitor inhibits acetylcholinesterase from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. For instance, edrophonium is a readily reversible acetylcholinesterase inhibitor that can be used to differentiate myasthenia gravis from cholinergic crisis. Edrophonium would improve muscle weakness in myasthenia gravis but not in cholinergic crisis.