Do opioids cause dilated pupils? The truth about drug effects on pupil size

October 10, 2025 •

4 min read

While many people associate drug use with dilated pupils, the opposite is true for opioids: they typically cause the pupils to constrict. The misconception about whether do opioids cause dilated pupils stems from confusion with other substances, but understanding this physiological response is critical for medical professionals and concerned individuals alike.

Quick Summary

Opioids cause pupils to constrict, a condition known as miosis, by stimulating the parasympathetic nervous system. Pupil dilation can, however, be a sign of opioid withdrawal. The presence of pinpoint pupils, along with other symptoms, can signal an overdose.

Key Points

Opioids cause constriction, not dilation: The key pharmacological effect of opioids on the pupils is to make them smaller, a condition known as miosis.
The mechanism involves the nervous system: Opioids stimulate the parasympathetic nervous system, which controls the muscles that constrict the pupils.
Withdrawal can cause dilation: Paradoxically, dilated pupils (mydriasis) can be a sign of opioid withdrawal due to an overactive sympathetic nervous system.
Pinpoint pupils can signal overdose: Pinpoint pupils, especially when non-reactive to light, are a critical indicator of a potential opioid overdose when combined with other symptoms like slowed breathing.
Other drugs cause dilation: Stimulants and hallucinogens are examples of drugs that typically cause pupil dilation, leading to the common misconception about all illicit substances.
Long-term use affects ocular health: Chronic opioid use and misuse can lead to serious eye infections, vision loss, and other ocular disorders.

The Opioid Effect: Why Pupils Constrict

Contrary to common belief, opioids do not cause dilated pupils. Instead, they produce miosis, or pupillary constriction, which often results in pupils so small they are referred to as "pinpoint pupils". This effect occurs because opioids influence the autonomic nervous system, which is responsible for regulating involuntary bodily functions, including pupil size.

The Role of the Nervous System

The autonomic nervous system consists of two opposing divisions: the sympathetic and parasympathetic systems. The sympathetic nervous system triggers the "fight or flight" response and causes the pupils to dilate (mydriasis), while the parasympathetic nervous system governs "rest and digest" functions and constricts the pupils.

Opioids, such as heroin, fentanyl, oxycodone, and morphine, bind to mu-opioid receptors in the central nervous system, including the brainstem. This action stimulates the parasympathetic nervous system, causing the sphincter pupillae muscle within the iris to contract. The sustained contraction of this muscle leads to the characteristic and persistent constriction of the pupils, regardless of ambient light levels.

The Conflicting Sign: Dilated Pupils During Opioid Withdrawal

An important contrast to the immediate effects of opioid use is the body's reaction during withdrawal. When a person who is physically dependent on opioids abruptly stops using them, the sympathetic nervous system becomes overactive. This surge in sympathetic activity triggers a range of withdrawal symptoms, including pupil dilation (mydriasis).

This phenomenon reflects the body's attempt to regain its balance in the absence of the opioid. The heightened sympathetic tone causes the iris dilator muscles to relax, allowing the pupils to widen. Therefore, while constricted pupils signal current opioid use, dilated pupils can be a key indicator of withdrawal, alongside other symptoms like anxiety, sweating, and nausea.

Opioids vs. Other Drugs: A Comparison of Pupil Effects

Not all drugs affect pupil size in the same way. The impact depends on the specific drug's mechanism of action on the central and autonomic nervous systems. A comparison table can help clarify the different effects:


Drug Type	Effect on Pupils	Mechanism	Example Drugs
Opioids	Constriction (Miosis)	Stimulates parasympathetic nervous system	Heroin, Morphine, Fentanyl, Oxycodone
Stimulants	Dilation (Mydriasis)	Increases levels of norepinephrine and dopamine	Cocaine, Methamphetamine, MDMA
Hallucinogens	Dilation or variability	Affects serotonin receptors, stimulating sympathetic activity	LSD, Psilocybin Mushrooms
Benzodiazepines	Constriction (Miosis)	Central nervous system depressant	Xanax, Valium
Opioid Withdrawal	Dilation (Mydriasis)	Heightened sympathetic nervous system activity	Cessation of opioids

The Critical Warning Sign: Pinpoint Pupils and Overdose

The presence of pinpoint pupils is a significant red flag in a potential opioid overdose situation. As opioids depress the central nervous system, they also slow down other critical bodily functions, most dangerously, breathing. When pinpoint pupils are observed in conjunction with other signs like slow or shallow breathing, unusual gurgling sounds, or unresponsiveness, it is a medical emergency.

In an overdose, the pupils may not respond to changes in light, a symptom known as being "fixed" or "non-reactive". While pinpoint pupils alone do not confirm an overdose, they are a crucial diagnostic sign for healthcare professionals, especially in a comatose or unresponsive patient.

Long-Term Ocular Effects of Opioid Use

Beyond the immediate changes in pupil size, chronic opioid use can lead to more serious and long-lasting effects on eye health.

Eye Infections: Misuse, particularly through intravenous injection, can lead to severe and rare eye infections, such as endogenous endophthalmitis. A compromised immune system and unsanitary injection practices allow bacteria or fungi to enter the bloodstream and spread to the eyes, which can cause permanent vision loss if left untreated.
Talc Retinopathy: For intravenous drug users, impurities like talc can get into the bloodstream and travel to the retina, causing blockages and damaging vision.
Other Ocular Issues: Long-term use has also been linked to other vision problems, including nystagmus (involuntary eye movements), glaucoma, and cataracts, although more research is needed on these connections.

Conclusion

In summary, the notion that opioids cause dilated pupils is a common and dangerous misconception. The pharmacological effect of opioids is exactly the opposite, causing pupillary constriction or miosis. This occurs by activating the parasympathetic nervous system. Understanding the difference between opioid-induced miosis and the pupil dilation that can occur during opioid withdrawal is vital for identifying a person's state and, in the case of an overdose, seeking emergency medical help. While pupil changes are an important indicator, they should be considered alongside other physical and behavioral signs to form an accurate assessment of a person's condition.

For more information on the effects of substance use on the body, consult reliable resources like the National Institute on Drug Abuse (NIDA).

National Institute on Drug Abuse

Frequently Asked Questions

Dilated pupils, or mydriasis, refers to pupils that are abnormally large, often a side effect of stimulants like cocaine. Pinpoint pupils, or miosis, are unusually small and constricted pupils, a key sign of opioid use.

Opioids cause pupils to become small by activating the parasympathetic nervous system. This system contracts the sphincter muscle in the iris, forcing the pupil to constrict.

Yes, but not typically during active use. Dilated pupils can occur during opioid withdrawal, as the body experiences an increase in sympathetic nervous system activity after the drug is stopped.

While pinpoint pupils are a highly reliable indicator of opioid use, they are not a definitive sign on their own. Other factors like light conditions, certain medical conditions, and other medications can also affect pupil size.

If you observe pinpoint pupils along with other signs of overdose, such as shallow breathing or unresponsiveness, you should call 911 immediately. An opioid overdose is a medical emergency.

During an opioid overdose, the pupils typically become very small (pinpoint) and may not react to light. In contrast, an overdose of a stimulant like cocaine would cause dilated pupils.

Yes, long-term or chronic opioid use can lead to various ocular issues, including severe eye infections, visual impairment, and other degenerative eye conditions.