An acetylcholinesterase inhibitor (often abbreviated AChEI) or anti-cholinesterase is a chemical or a drug  that inhibits the acetylcholinesterase enzyme from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. Acetylcholinesterase inhibitors are classified as reversible, irreversible, or quasi-irreversible (also called pseudo-irreversible).