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In page Physostigmine:

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Physostigmine acts by interfering with the metabolism of acetylcholine. It is a covalent (reversible - bond hydrolyzed and released) inhibitor of acetylcholinesterase, the enzyme responsible for the breakdown of acetylcholine in the synaptic cleft of the neuromuscular junction.[1] It indirectly stimulates both nicotinic and muscarinic acetylcholine receptors. Physostigmine has an LD50 of 3 mg/kg in mice.