When the black center of your eyes appears larger than usual, this condition is termed mydriasis. Normally, pupils expand and contract to regulate the amount of light entering the eye. However, certain medications can cause this dilation, even in brightly lit environments, by interfering with pupil size control.
How Medications Influence Pupil Size
The size of your pupils is controlled by a balance within the autonomic nervous system. Two main branches, the sympathetic and parasympathetic nervous systems, oppose to adjust pupil diameter. The parasympathetic system, associated with “rest and digest” functions, causes the pupil to constrict by activating the iris sphincter muscle. Conversely, the sympathetic nervous system, known for its “fight or flight” response, triggers pupil dilation by contracting the iris dilator muscle.
Medications influence pupil size by disrupting this balance. Some drugs mimic the sympathetic nervous system, enlarging pupils. Others block the parasympathetic nervous system, preventing constriction. This nerve signal interference results in sustained pupil widening.
Specific Medication Classes Causing Dilation
Several medication categories cause pupil dilation by affecting nervous system control. A primary group is anticholinergic medications. These drugs block acetylcholine, a neurotransmitter causing pupil constriction. Examples include atropine and scopolamine, used in eye drops for examinations, and certain antihistamines like diphenhydramine. Tricyclic antidepressants also possess anticholinergic properties, contributing to dilation.
Another class, sympathomimetic medications, stimulates the sympathetic nervous system, widening pupils. Common decongestants like pseudoephedrine and phenylephrine fall into this category. Stimulant medications, including amphetamines and methylphenidate, also increase neurotransmitter levels like norepinephrine and dopamine, activating iris dilator muscles. Eye drops known as mydriatics often contain phenylephrine or tropicamide, working by stimulating the dilator muscle or blocking the constricting muscle.
Beyond these, some antidepressants, including selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), can cause pupil dilation, less commonly than tricyclics. This effect relates to their influence on serotonin levels, which relax the iris sphincter muscle.
Certain recreational substances induce pupil dilation by acting on similar pathways. Cocaine and amphetamines, for instance, block norepinephrine reuptake, increasing stimulation and dilation. Hallucinogens like LSD and MDMA affect serotonin receptors, resulting in pupil enlargement.
When to Seek Medical Advice
While medication-induced pupil dilation can be a common, expected side effect (e.g., after an eye examination), it sometimes warrants medical attention. Consult a healthcare professional if pupil dilation occurs suddenly, without clear reason, or persists. This is especially important if dilation is unequal (anisocoria) or accompanied by other concerning symptoms.
Symptoms prompting immediate medical evaluation with dilated pupils include sudden vision changes, eye pain, headache, light sensitivity, dizziness, or confusion. These could indicate a serious underlying issue, such as head injury, stroke, or other neurological conditions. Do not discontinue prescribed medication without speaking to your doctor or pharmacist, as they can assess the situation and provide guidance.