What Substances Cause Pupils to Dilate? An Exploration of Mydriatics, Drugs, and Pharmacology

October 9, 2025 •

4 min read

The pupil's size is controlled by the delicate balance of the autonomic nervous system, making it a reliable indicator of an individual's physiological state. Consequently, a wide range of substances and medications can alter this balance, leading to mydriasis, or pupil dilation. Understanding what substances cause pupils to dilate is crucial for both medical and public health contexts.

Quick Summary

A diverse array of substances can cause pupil dilation by interfering with the sympathetic and parasympathetic nervous systems. This includes prescribed mydriatics for eye exams, anticholinergics, stimulants, and recreational hallucinogens. Effects can range from temporary light sensitivity to indications of drug use or underlying medical conditions.

Key Points

Mydriasis Defined: Mydriasis is the medical term for pupil dilation, or the widening of the pupils.
Nervous System Balance: Substances cause pupils to dilate by interfering with the autonomic nervous system's control of the iris muscles.
Medical Dilation: Mydriatics like atropine and tropicamide are medically used to dilate pupils for eye exams and surgery.
Stimulants and Mydriasis: Recreational stimulants such as cocaine, methamphetamine, and MDMA cause pupil dilation by increasing norepinephrine.
Hallucinogens' Effect: Psychedelic substances like LSD and psilocybin activate serotonin and adrenergic receptors, leading to significant mydriasis.
Anticholinergic Action: Many over-the-counter and prescription drugs, including anticholinergics and some antidepressants, can cause dilation by blocking pupil-constricting signals.
Beyond Drugs: Non-pharmacological factors such as emotional states, changes in lighting, and medical conditions like head injuries can also cause pupils to dilate.

The Pharmacology of Pupil Dilation

The size of the pupil is regulated by two opposing muscles within the iris: the sphincter muscle, which constricts the pupil, and the dilator muscle, which widens it. The sphincter is controlled by the parasympathetic nervous system, while the dilator is controlled by the sympathetic nervous system. Mydriasis, or pupil dilation, occurs when a substance either stimulates the sympathetic nervous system or inhibits the parasympathetic nervous system. By disrupting this delicate neural balance, various chemicals can cause the pupil to enlarge, irrespective of ambient light conditions. Medical professionals, for example, often induce mydriasis for diagnostic purposes, while other substances may have this effect as an unintended side effect or a result of recreational use.

Medications and Their Mydriatic Effects

Ophthalmic Mydriatics

These are a class of eye drops specifically formulated to induce temporary pupil dilation for ophthalmological examinations and procedures. Examples include:

Tropicamide: A fast-acting anticholinergic that is a standard for routine dilation.
Phenylephrine: An adrenergic agonist that stimulates the iris dilator muscle. Often used in combination with anticholinergics to achieve maximum dilation.
Atropine: A potent, longer-lasting anticholinergic sometimes used for treating specific inflammatory eye conditions or amblyopia.

Anticholinergic Drugs

Systemic anticholinergic medications block the action of acetylcholine, which is the neurotransmitter responsible for parasympathetic control. By blocking this signal, the sphincter muscle is relaxed, leading to dilation. Common examples include prescription drugs used for motion sickness (e.g., scopolamine), overactive bladder, and certain respiratory conditions.

Other Prescription Drugs

Several other prescribed medications, often affecting the central nervous system, can have mydriasis as a side effect. These include:

Selective Serotonin Reuptake Inhibitors (SSRIs): Certain antidepressants can affect neurotransmitter levels that influence pupil size.
Stimulant Medications for ADHD: Amphetamines like Adderall increase the activity of the sympathetic nervous system.
Antihistamines and Decongestants: Some over-the-counter allergy and cold medications can also have anticholinergic or sympathomimetic effects.
Benzodiazepines: These depressants can cause dilated pupils at recreational or higher doses.

Recreational Substances That Cause Dilated Pupils

Stimulants

Recreational stimulants intensify the 'fight or flight' response, causing a surge in norepinephrine and dopamine. This leads to the characteristic dilation of pupils alongside increased heart rate and energy.

Cocaine: Blocks the reuptake of norepinephrine, leading to a strong mydriatic effect.
Methamphetamine and Amphetamines: Stimulate the release of norepinephrine.
MDMA (Ecstasy): Increases serotonin and norepinephrine, causing pupil dilation.

Hallucinogens and Psychoactives

These substances profoundly affect perception and often cause significant pupil dilation, a visual hallmark of their use.

LSD (Acid) and Psilocybin (Magic Mushrooms): Interact with serotonin receptors, leading to heightened sympathetic activity and visual changes.
Mescaline: A psychedelic derived from cacti, it also causes pronounced mydriasis.

Other Substances

Some other substances of abuse can also cause mydriasis or contribute to it:

Opioid Withdrawal: While opioids themselves cause pinpoint pupils (miosis), the withdrawal process can lead to mydriasis as the body rebounds from their constricting effects.
Cannabis: Marijuana can cause pupil dilation in some individuals, though effects can vary depending on the person and dose.

Comparison of Mydriatic-Causing Substances


Substance Category	Primary Mechanism	Common Examples	Typical Context of Use
Ophthalmic Mydriatics	Block parasympathetic/activate sympathetic signaling in the eye's iris muscles	Atropine, Tropicamide, Phenylephrine	Medical eye exams, procedures, and treatments
Systemic Anticholinergics	Block muscarinic acetylcholine receptors throughout the body, including eye muscles	Scopolamine, certain COPD drugs	Motion sickness, systemic conditions
Stimulants	Increase norepinephrine and dopamine in the nervous system	Cocaine, Methamphetamine, MDMA, ADHD meds (Adderall)	Recreational use, legitimate ADHD treatment
Hallucinogens	Interact with serotonin and adrenergic receptors, affecting visual pathways	LSD, Psilocybin (mushrooms), Mescaline	Recreational use

Other Causes and Considerations

Beyond pharmacological agents, mydriasis can occur due to non-substance-related factors. A person's pupils naturally dilate in low-light conditions to allow more light to enter the eye. Emotional states, such as excitement, arousal, or fear, can also activate the sympathetic nervous system and cause dilation. Certain medical conditions can also be responsible, including brain injury, stroke, tumors, or damage to the third cranial nerve. It is important to consider the full context of a person's physical state, environment, and recent exposure to substances when evaluating the cause of dilated pupils. In cases of injury or if other symptoms are present, it is crucial to seek immediate medical attention.

Conclusion

From medically necessary dilating eye drops used during routine eye exams to the powerful effects of recreational drugs, a variety of substances can cause pupils to dilate by influencing the delicate balance of the autonomic nervous system. Understanding the specific pharmacological mechanisms behind mydriasis—whether it is the result of anticholinergic action, sympathetic stimulation, or central nervous system effects—is essential for medical diagnosis and public awareness. While temporary dilation from medication is common and harmless, prolonged or unexplained mydriasis, particularly when accompanied by other symptoms, may be a sign of a more serious underlying issue requiring prompt attention. For more information on medications and their effects, resources like the Mayo Clinic can provide additional details and guidance.

Frequently Asked Questions

Identifying the cause of dilated pupils solely based on visual observation can be difficult, as many factors can cause mydriasis. However, if pupils are significantly dilated and unresponsive to light, especially when accompanied by other signs like altered behavior, rapid heart rate, or slurred speech, substance use may be a factor.

No, not all recreational drugs cause pupil dilation. For example, opioids like heroin and morphine typically cause miosis, or pinpoint pupils. The effect depends on the specific drug and its interaction with the nervous system.

Eye doctors use mydriatic eye drops to dilate pupils so they can get a better view of the internal structures of the eye, such as the retina and optic nerve. This is essential for detecting or monitoring eye diseases like glaucoma and macular degeneration.

Yes, some over-the-counter medications can cause mydriasis as a side effect. Common culprits include certain antihistamines, cold medications, and decongestants, which may have anticholinergic or sympathomimetic properties.

The duration of drug-induced pupil dilation depends on the substance, dosage, and individual metabolism. For example, mydriatic eye drops used by an eye doctor typically wear off within 4 to 6 hours. The effects of recreational drugs like LSD can last for many hours.

While temporary dilation from an eye exam is not dangerous, prolonged or unexplained mydriasis can be cause for concern. It can lead to light sensitivity and blurry vision, and in rare cases, could signal a more serious underlying medical issue like a head injury or aneurysm.

Mydriasis is the dilation or widening of the pupils, while miosis is the constriction or narrowing of the pupils. Different substances can cause either effect, depending on how they affect the nervous system's control of the iris muscles.