Myosis (Miosis): Why Your Pupils Are Small and When to Act

Notice your pupils are unusually small in one or both eyes? That's called myosis (more commonly spelled miosis). Small pupils can be harmless, like after bright light, or a sign of a medical issue. This guide explains common causes, what symptoms to watch for, and simple steps you can take right away.

Common causes of myosis

Bright light and normal reactions: Your pupils shrink to protect the eye in bright environments — that's normal and temporary. Drugs: Opioids (like morphine, codeine) often cause pinpoint pupils. Certain eye drops, such as pilocarpine or dorzolamide-related treatments for glaucoma, can also constrict pupils. Nervous system issues: Conditions affecting the brainstem or cranial nerves can lead to one-sided or both-sided miosis. Horner's syndrome, for example, causes a small pupil on the affected side plus drooping eyelid and decreased sweating. Head injury, stroke, or tumors can show up with pupil changes too.

Symptoms that mean you should see a doctor

If small pupils come with headache, sudden vision changes, drooping eyelid, severe dizziness, confusion, or unequal pupil sizes, get medical help quickly. Also seek care if pupils stay tiny after using new medication, or after a head injury. If you use opioids and notice pinpoint pupils plus drowsiness or slow breathing, call emergency services—this can signal an overdose.

For less urgent cases, make an appointment with your primary care doctor or an eye specialist. They will ask about recent medications, injuries, and other symptoms. A simple eye exam checks pupil size and reaction to light. Doctors may test nerve pathways, order imaging (CT or MRI) if a neurologic cause is suspected, or review your meds.

Treatment depends on the cause. If a medication is the culprit, stopping or switching drugs under medical guidance often fixes it. Eye drops that dilate the pupil can be used short-term for discomfort. For nerve-related causes like Horner's syndrome, doctors focus on finding the underlying problem—sometimes imaging or blood tests are needed. Emergency treatment is crucial for causes like stroke or severe head trauma.

Simple steps you can take now: check both pupils in bright and dim light, note if only one pupil is small, list recent medications or eye drops, and watch for other symptoms like drooping eyelid or vision loss. Avoid driving if your vision feels off. Bring a clear list of medicines to your appointment—this speeds up diagnosis.

Small pupils are often harmless, but they can be a clue to something more serious. Paying attention, acting quickly when warning signs appear, and getting a proper exam will get you the right care faster.