An acetylcholinesterase inhibitor (often abbreviated AChEI) or anti-cholinesterase is a chemical that inhibits the acetylcholinesterase enzyme from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. Reversible, quasi-irreversible (or pseudirreversible in some sources) and irreversible inhibitors exist.
Actions on the neuromuscular junction will result in prolonged muscle contraction.
Administration of reversible cholinoesterase inhibitors is contraindicated with those that have urinary retention due to obstruction.
When used in the central nervous system to alleviate neurological symptoms, such as rivastigmine in Alzheimer's disease, all cholinesterase inhibitors require doses to be increased gradually over several weeks, and this is usually referred to as the titration phase. Many other types drug treatments may require a titration or stepping up phase. This strategy is used to build tolerance to adverse events or to reach a desired clinical effect. 
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