Pupil dilation, medically termed mydriasis, occurs when the black center of the eye widens, allowing more light to reach the retina. This change in size results from the iris muscles contracting or relaxing in response to stimuli like light, emotion, or chemical substances. Since the iris muscles are controlled by the nervous system, any drug that interferes with these chemical signals can cause dilation. Many different classes of substances, from topical eye drops to systemic medications and recreational drugs, can produce this effect.
How the Pupil Size is Controlled
The pupil’s size is controlled by two opposing sets of smooth muscles within the iris. The sphincter pupillae muscle encircles the pupil; when it contracts, it reduces the pupil’s size (miosis). The dilator pupillae muscle consists of fibers that radiate outward, and its contraction pulls the pupil open, causing mydriasis.
These two muscles are regulated by the autonomic nervous system, which manages involuntary bodily functions. The parasympathetic nervous system signals the sphincter muscle to contract, constricting the pupil. Conversely, the sympathetic nervous system, responsible for the “fight or flight” response, signals the dilator muscle to contract, causing the pupil to widen.
Drugs cause pupil dilation by shifting the balance of this system. They either stimulate the sympathetic pathway or, more commonly, block the action of the parasympathetic pathway. Blocking the parasympathetic signal prevents the sphincter muscle from constricting the pupil, allowing the dilator muscle’s influence to dominate. Substances that activate the sympathetic system directly stimulate the dilator muscle to open the pupil.
Medications Used for Eye Examinations
Some drugs are specifically formulated to induce mydriasis for ophthalmic examinations. These topical eye drops allow eye care professionals to get a clear view of the retina and optic nerve. The two main types of drops achieve dilation by working through different nervous system pathways.
One category is anticholinergics, also called cycloplegics, which include atropine, cyclopentolate, and tropicamide. These agents block acetylcholine, the neurotransmitter that signals the sphincter muscle to constrict. By paralyzing the constrictor muscle, these drops allow the pupil to dilate widely.
The second type is sympathomimetics, such as phenylephrine, which mimic the effect of adrenaline. These drugs stimulate the dilator muscle to contract, actively pulling the pupil open. To achieve maximum dilation, professionals often use a combination of a sympathomimetic and an anticholinergic agent. This dual approach ensures the constricting muscle is relaxed and the dilating muscle is stimulated.
Common Prescription and OTC Side Effects
Pupil dilation is a common, unintended side effect of many systemic medications taken for non-eye-related conditions. These drugs affect the autonomic nervous system after being absorbed into the bloodstream, resulting in noticeable pupil widening. Antidepressants are a frequent cause, particularly tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs). Tricyclic antidepressants often have anticholinergic properties that inhibit parasympathetic control, while SSRIs affect serotonin levels, which influence pupil size.
Many over-the-counter (OTC) cold and allergy medications can also lead to mydriasis. First-generation antihistamines, such as diphenhydramine, commonly possess strong anticholinergic properties, causing dilation. Decongestants containing pseudoephedrine or phenylephrine stimulate the sympathetic nervous system throughout the body, including the eye’s dilator muscle.
Prescription stimulants used to treat Attention-Deficit/Hyperactivity Disorder (ADHD), such as amphetamine salts or methylphenidate, also cause dilation. These medications increase norepinephrine and dopamine levels in the brain, activating the sympathetic “fight or flight” response. Other drugs, including certain anti-nausea and anti-seizure medications, may exhibit this effect due to their influence on the same nervous system pathways.
Recreational Substances That Cause Dilation
Pronounced mydriasis is a characteristic physical sign associated with recreational substance use. These substances typically act as powerful central nervous system stimulants, leading to massive activation of the body’s sympathetic response. Stimulants like cocaine, methamphetamine, and MDMA (ecstasy) cause the release of high levels of norepinephrine and dopamine. This surge of neurotransmitters over-activates the dilator muscle, resulting in significant pupil widening.
Hallucinogenic drugs, including LSD (lysergic acid diethylamide) and psilocybin, are also strongly associated with mydriasis. These substances primarily interact with serotonin receptors in the brain, profoundly influencing the autonomic nervous system and triggering pupil dilation. The resulting dilation from these powerful stimulants and hallucinogens is often far more extreme than that seen with common prescription medications.
It is important to understand that severe or sudden pupil dilation in the context of recreational substance use can be a sign of toxicity or physiological distress. If mydriasis is accompanied by confusion, rapid heart rate, high body temperature, or other signs of overdose, immediate medical attention is necessary. Unlike temporary side effects from prescribed drugs, extreme dilation from these substances may indicate a medical emergency.