Pupils are the black centers of your eyes, openings that control the amount of light reaching the retina. They adjust in size, becoming smaller in bright conditions and larger in dim light, a natural reflex to optimize vision. When pupils become unusually large, a condition known as mydriasis occurs. This expansion can be a temporary response to various internal and external factors, including the presence of certain medications and other substances.
How Medications and Substances Affect Pupil Size
The size of your pupils is regulated by the autonomic nervous system. This system has two main branches: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic system is responsible for the “fight or flight” response. It causes pupil dilation by stimulating the dilator pupillae muscle within the iris.
Conversely, the parasympathetic nervous system promotes “rest and digest” activities, leading to pupil constriction. This occurs by stimulating the sphincter pupillae muscle. Drugs and substances can influence pupil size by either activating the sympathetic nervous system or by blocking the effects of the parasympathetic nervous system. When the sympathetic pathway is enhanced or the parasympathetic pathway is inhibited, the dilator muscle contracts, leading to an increase in pupil diameter.
Common Medicinal Drugs That Dilate Pupils
Several medicinal drugs, both prescription and over-the-counter, can lead to pupil dilation.
Ophthalmic Medications
These are eye drops used during eye examinations. Drugs like atropine, cyclopentolate, and phenylephrine are applied directly to the eye to temporarily paralyze the muscles that constrict the pupil or stimulate the dilator muscle, allowing for a better view of the retina. The effects last for several hours.
Anticholinergic Medications
These medications block the action of acetylcholine, a neurotransmitter involved in parasympathetic activity. Since the parasympathetic system normally constricts the pupil, blocking it results in dilation. These drugs are found in various medications, including some cold and allergy remedies, motion sickness patches (like scopolamine), and certain bladder control medications. Their systemic effects can lead to dilated pupils as a side effect.
Antidepressants
Some antidepressant medications, particularly older tricyclic antidepressants (TCAs) such as amitriptyline or imipramine, can also cause mydriasis due to their anticholinergic properties. Even some newer selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) may occasionally cause mild dilation, though less consistently than TCAs. This effect is related to their broader impact on neurotransmitter systems.
Decongestants
These sympathomimetic agents work by stimulating the sympathetic nervous system. Oral medications and nasal sprays containing ingredients like pseudoephedrine or phenylephrine can constrict blood vessels in the nasal passages, but they can also activate adrenergic receptors in the eye, leading to pupil dilation. This systemic effect means that even a nasal spray can influence pupil size.
Antihistamines
Older, sedating antihistamines, such as diphenhydramine (found in many allergy and sleep aids), possess anticholinergic properties. They block the neurotransmitter acetylcholine, reducing parasympathetic influence on the pupil. This allows the sympathetic system to dominate, resulting in pupil enlargement as a side effect.
Illicit Substances That Dilate Pupils
Beyond medicinal compounds, several illicit substances cause pupil dilation.
Stimulants
Stimulants like cocaine, amphetamines (including methamphetamine), and MDMA (ecstasy) are examples. These substances increase the release of neurotransmitters such as dopamine, norepinephrine, and serotonin in the brain. The surge in norepinephrine overactivates the sympathetic nervous system, leading to pupil dilation.
Hallucinogens
Hallucinogens, including LSD (lysergic acid diethylamide) and psilocybin (the active compound in magic mushrooms), also induce mydriasis. Their mechanism involves interacting with serotonin receptors in the brain. This interaction can lead to increased sympathetic nervous system activity, resulting in dilated pupils. The degree of dilation correlates with the substance’s potency and the individual’s sensitivity.
Cannabis
Cannabis can sometimes cause changes in pupil size. Its effects on the body involve cannabinoid receptors. While some users might experience dilation, others may not notice any change, making it less predictable compared to other substances.
When to Be Concerned About Dilated Pupils
While dilated pupils can be a side effect of medication or environmental changes, certain circumstances warrant medical attention. Unequal pupil size, a condition known as anisocoria, is a concern, especially if it appears suddenly or is new for an individual. This asymmetry can signal an underlying neurological issue or injury.
Seek medical advice if pupil dilation occurs suddenly without a clear cause, such as new medications or eye drops. If dilated pupils are accompanied by other symptoms like a severe headache, blurry or double vision, dizziness, confusion, nausea, or unusual sensitivity to light, it may indicate a more serious condition. Following a head trauma, any change in pupil size, especially unequal dilation, requires immediate emergency care as it may suggest brain injury.