A parasympathomimetic drug, sometimes called a cholinomimetic drug^[1] or cholinergic receptor stimulating agent,^[2] is a substance that stimulates the parasympathetic nervous system (PSNS).^[3][2] These chemicals are also called cholinergic drugs because acetylcholine (ACh) is the neurotransmitter used by the PSNS.^[1][4] Chemicals in this family can act either directly by stimulating the nicotinic or muscarinic receptors (thus mimicking acetylcholine), or indirectly by inhibiting cholinesterase, promoting acetylcholine release, or other mechanisms.^[5] Common uses of parasympathomimetics include glaucoma, Sjögren syndrome and underactive bladder.^[6]

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Some chemical weapons such as sarin or VX, non-lethal riot control agents such as tear gas, and insecticides such as diazinon fall into this category.

For a cholinergic agent, the following criteria describe the structure activity relationship:

1. Ing's Rule of 5: there should be no more than five atoms between the nitrogen and the terminal hydrogen for muscarinic (or cholinergic) activity;
2. the molecule must possess a nitrogen atom capable of bearing a positive charge, preferably a quaternary ammonium salt;
3. for maximum potency, the size of the alkyl groups substituted on the nitrogen should not exceed the size of a methyl group;
4. the molecule should have an oxygen atom, preferably an ester-like oxygen capable of participating in a hydrogen bond;
5. there should be a two-carbon unit between the oxygen atom and the nitrogen atom.

• Sympathomimetic drug
• Parasympatholytics
• sympatholytics