Which drug causes miosis? Understanding Medication-Induced Pupil Constriction

October 9, 2025 •

2 min read

According to Drugs.com, pinpoint pupils, or miosis, are most commonly associated with opioid intoxication, but several other drugs, including certain antipsychotics, antihypertensives, and toxins, can also cause this sign. Understanding which drug causes miosis is vital for proper diagnosis and management, particularly in emergency situations.

Quick Summary

This article explores the various medications and toxins that can induce miosis, detailing their pharmacological mechanisms. It covers prescription eye drops, opioids, pesticides, and other drug classes known to constrict pupils, and discusses associated risks.

Key Points

Opioids are a leading cause of miosis: Opioids like morphine, heroin, and fentanyl are notorious for causing pinpoint pupils, especially in overdose situations.
Cholinergic medications cause miosis by stimulating the parasympathetic system: Drugs like pilocarpine (eye drops) and donepezil (Alzheimer's) cause pupils to constrict by acting on the cholinergic system.
Pesticides can cause severe miosis: Organophosphate pesticides and nerve agents are potent anticholinesterase inhibitors, leading to excessive stimulation and pronounced miosis.
Miosis can be a sign of a medical emergency: While some drug-induced miosis is benign, it can signal a dangerous overdose or other conditions like a stroke, requiring immediate medical attention.
Treatment depends on the cause: For drug-induced miosis, treatment may involve stopping the offending medication, administering an antidote (like naloxone for opioids), or managing the underlying condition.
Several other drugs can cause pupil constriction: Antipsychotics, clonidine (for blood pressure), and certain antiemetics are also associated with miosis, though it is less common.

The Autonomic System's Role in Pupil Size

Pupil size is controlled by the autonomic nervous system, which has sympathetic and parasympathetic branches. The sympathetic system causes dilation, while the parasympathetic system causes constriction (miosis). Drugs that induce miosis typically stimulate the parasympathetic pathway or inhibit the sympathetic pathway.

Major Drug Classes That Cause Miosis

Opioids and Other Central Nervous System Depressants

Opioids are well-known for causing miosis by activating mu-opioid receptors, which stimulates the parasympathetic nervous system. This effect is a key indicator of opioid toxicity or overdose and is often seen with respiratory depression. Common opioids causing miosis include prescription painkillers, heroin, fentanyl, and buprenorphine. Other CNS depressants like clonidine and some barbiturates can also cause miosis in overdose.

Cholinergic Agents and Toxic Exposures

Cholinergic agents mimic or enhance the action of acetylcholine, a neurotransmitter in the parasympathetic system. Direct-acting agents, such as pilocarpine eye drops for glaucoma, directly stimulate iris sphincter muscles. Indirect-acting agents, or anticholinesterase inhibitors, block the breakdown of acetylcholine, increasing its levels and causing miosis. Examples include donepezil for Alzheimer's and neostigmine for myasthenia gravis, with miosis being more common in overdose. Potent anticholinesterase inhibitors like organophosphate pesticides and nerve agents cause severe miosis due to excessive parasympathetic stimulation.

Other Medications with Miotic Effects

Some other drugs can cause miosis as a side effect. These include certain older and newer antipsychotics like olanzapine, antiemetics such as droperidol, and serotonergic agents like buspirone.

Comparison of Drug-Induced Miosis


Drug Class	Examples	Primary Mechanism	Clinical Context
Opioids	Heroin, Fentanyl, Morphine	Stimulates mu-opioid receptors, activating parasympathetic nerves.	Overdose, chronic use; often associated with respiratory depression.
Direct Cholinergic Agonists	Pilocarpine, Carbachol	Directly stimulates muscarinic receptors on the iris sphincter muscle.	Eye drops for glaucoma or surgical procedures.
Anticholinesterase Inhibitors	Donepezil, Organophosphate Pesticides	Inhibits acetylcholinesterase, increasing acetylcholine levels.	Overdose, toxic exposure, Alzheimer's disease treatment.
Antipsychotics	Olanzapine, Chlorpromazine	Less common; potential alpha-1 adrenergic receptor blockade.	Medication side effect; less pronounced than opioids.
Clonidine	Clonidine	Decreases sympathetic outflow from the central nervous system.	Overdose, chronic use for high blood pressure.

Diagnosis and Management

Abnormal miosis, especially with other symptoms, warrants medical consultation. Diagnosis involves reviewing medication history and physical examination, potentially with tests to exclude other causes like stroke. Treatment for drug-induced miosis may involve discontinuing the medication, using antidotes like naloxone for opioid overdose or atropine for organophosphate poisoning, or managing the underlying condition.

Conclusion

Miosis can be a key indicator of drug exposure, from therapeutic eye drops to dangerous overdoses of opioids or pesticides. Various substances can cause miosis by affecting the autonomic nervous system. Recognizing which drug causes miosis and its mechanism is crucial for prompt diagnosis and effective treatment, ensuring patient safety. It's essential to seek medical advice for unexpected changes in pupil size. For further information on constricted pupils, refer to the Cleveland Clinic overview.

Frequently Asked Questions

The most common and clinically significant class of drugs associated with miosis, or pinpoint pupils, is opioids. This includes both prescription pain relievers like morphine and oxycodone, as well as illicit drugs like heroin and fentanyl.

No, not all glaucoma medications cause pupil constriction. While miotic eye drops like pilocarpine are used to constrict the pupil to increase aqueous humor drainage and reduce eye pressure, other glaucoma treatments work differently without this effect.

Drug-induced miosis typically affects both pupils symmetrically, though some drug exposures may affect only one eye. A stroke, particularly a pontine stroke, can cause miosis, but it is often accompanied by other neurological signs. A healthcare provider can distinguish between these causes through examination and other diagnostic tests.

Yes, some antipsychotic medications have been associated with miosis, including older agents and some second-generation drugs like olanzapine. This is often a less prominent side effect compared to opioid-induced miosis.

Yes, miosis (pinpoint pupils) is a classic sign of opioid overdose and is frequently observed alongside other critical symptoms like respiratory depression and decreased consciousness. Recognizing this is crucial for emergency intervention.

Treatment for drug-induced miosis depends on the specific cause. In the case of an opioid overdose, the antidote naloxone can be administered. For medications causing a side effect, a doctor might change the prescription. In poisoning cases, specific antidotes or supportive care may be necessary.

Organophosphate pesticides cause miosis by inhibiting the acetylcholinesterase enzyme, which leads to a buildup of acetylcholine. This excessive acetylcholine overstimulates the parasympathetic nervous system, causing profound and sustained pupil constriction.