How does pilocarpine cause miosis?

October 7, 2025 •

3 min read

An estimated 4.2 million Americans have glaucoma, a condition where the medication pilocarpine can be used to help manage eye pressure. Understanding how does pilocarpine cause miosis, or pupil constriction, is key to understanding its therapeutic effect.

Quick Summary

Pilocarpine is a parasympathomimetic drug that directly stimulates muscarinic receptors on the eye's iris sphincter muscle, causing the muscle to contract and induce miosis (pupil constriction).

Key Points

Direct Agonist: Pilocarpine is a direct-acting parasympathomimetic that mimics acetylcholine to cause its effects.
Miosis Mechanism: It causes miosis (pupil constriction) by directly stimulating M3 muscarinic receptors on the iris sphincter muscle, causing it to contract.
Glaucoma Treatment: It aids in glaucoma by contracting the ciliary muscle, which opens the trabecular meshwork to improve aqueous humor outflow and lower eye pressure.
Drug Class: Pilocarpine is classified as a miotic and a cholinergic (or muscarinic) agonist.
Clinical Uses: Key applications include treating glaucoma, presbyopia, and reversing diagnostic pupil dilation.
Dual Action: The drug acts on both the iris sphincter muscle to cause miosis and the ciliary muscle to aid accommodation and reduce intraocular pressure.
Side Effects: Common side effects stem from its cholinergic nature and include blurred vision, brow ache from muscle spasms, and systemic effects like sweating.

Understanding Pilocarpine's Role in Ocular Health

Pilocarpine is a medication that has been used in ophthalmology for over a century, primarily for its ability to reduce intraocular pressure (IOP) in patients with glaucoma. It belongs to a class of drugs known as miotics, or parasympathomimetic agents, which means it mimics the effects of the parasympathetic nervous system. The core of its function lies in its ability to constrict the pupil, a process called miosis. This action is not only a visible effect but also a key part of its therapeutic mechanism for certain eye conditions.

The Autonomic Nervous System and Pupil Control

To understand how pilocarpine works, it's essential to first understand how the pupil is controlled. The size of the pupil is regulated by two muscles within the iris, which are in turn controlled by the autonomic nervous system:

Iris Sphincter Muscle (Sphincter Pupillae): This circular muscle is innervated by the parasympathetic nervous system. When it contracts, the pupil constricts (miosis).
Iris Dilator Muscle: This radial muscle is controlled by the sympathetic nervous system. Its contraction leads to pupil dilation (mydriasis).

Pilocarpine's action is focused on the parasympathetic pathway, specifically mimicking the neurotransmitter acetylcholine.

The Molecular Mechanism: How Does Pilocarpine Cause Miosis?

Pilocarpine is a direct-acting cholinergic agonist. It binds to and stimulates muscarinic acetylcholine receptors, primarily the M3 subtype on the iris sphincter muscle. When administered, typically as an eye drop, it causes the iris sphincter muscle to contract, leading to miosis (pupil constriction). This effect usually occurs within an hour and can last for a day.

Dual Action: Miosis and Ciliary Muscle Contraction

Pilocarpine also stimulates M3 receptors on the ciliary muscle. This contraction affects the lens shape, potentially improving near vision (spasm of accommodation), and increases tension on the scleral spur. This opens the trabecular meshwork, increasing aqueous humor outflow and lowering intraocular pressure, which is crucial for treating open-angle glaucoma. In angle-closure glaucoma, miosis helps by pulling the iris away from the drainage angle.

Clinical Applications

Pilocarpine is used in several clinical scenarios:

Glaucoma: It treats primary open-angle glaucoma, ocular hypertension, and acute angle-closure glaucoma. It's no longer a first-line treatment but remains useful.
Presbyopia: A low-dose formulation is FDA-approved to treat age-related blurry near vision by creating a "pinhole" effect through miosis.
Reversing Mydriasis: It can constrict the pupil after diagnostic dilation.
Xerostomia (Dry Mouth): Oral pilocarpine increases saliva production by stimulating salivary gland receptors.

Comparison of Pilocarpine with Other Ocular Drugs


Feature	Pilocarpine	Physostigmine	Atropine
Drug Class	Direct-acting muscarinic agonist	Indirect-acting parasympathomimetic (AChE inhibitor)	Muscarinic antagonist
Mechanism	Directly stimulates muscarinic receptors on the iris sphincter and ciliary muscle.	Inhibits acetylcholinesterase, increasing local acetylcholine concentrations at the synapse.	Blocks acetylcholine from binding to muscarinic receptors, preventing muscle contraction.
Effect on Pupil	Miosis (constriction)	Miosis (constriction)	Mydriasis (dilation)
Primary Use	Glaucoma, Presbyopia, Xerostomia	Reversal of anticholinergic toxicity; less common in ophthalmology now.	Pupil dilation for exams, treatment of uveitis, antidote for cholinergic poisoning.

Side Effects and Considerations

Common ocular side effects of pilocarpine include brow ache, blurred vision, and irritation. Difficulty seeing in dim light can occur due to pupil constriction. Systemic effects like sweating and nausea are possible from systemic absorption. Retinal detachment is a rare, serious side effect.

Conclusion

Pilocarpine induces miosis by acting as a cholinergic agonist that directly stimulates M3 muscarinic receptors on the iris sphincter muscle, causing it to contract and reduce pupil size. This same action on the ciliary muscle contributes to its use in lowering intraocular pressure for glaucoma and treating presbyopia.

For more detailed information, consult the StatPearls article on Pilocarpine: https://www.ncbi.nlm.nih.gov/books/NBK556128/

Frequently Asked Questions

Miosis is the medical term for the constriction or narrowing of the pupil of the eye.

Pilocarpine is a parasympathomimetic agent, meaning it mimics the effects of the parasympathetic nervous system, such as stimulating acetylcholine receptors.

When administered as an eye drop, pilocarpine typically begins to work within an hour.

The main uses are to reduce high intraocular pressure in glaucoma and to treat presbyopia (age-related blurry near vision).

Yes, common side effects include headache, brow ache, blurred vision, and eye irritation. Systemic side effects like sweating and increased salivation can also occur.

Pilocarpine contracts the ciliary muscle, which increases tension on the scleral spur and opens the trabecular meshwork. This action improves the drainage of aqueous humor from the eye, lowering intraocular pressure.

Pilocarpine is a muscarinic agonist that constricts the pupil (miosis), while atropine is a muscarinic antagonist that blocks these receptors to dilate the pupil (mydriasis).

Yes, pilocarpine can cause blurred vision, accommodative spasm (difficulty changing focus), and trouble seeing in dim or dark environments due to pupil constriction.