Myosis: Why your pupils are smaller and what to do

If you or someone else notices unusually small pupils, that's called "myosis" (often spelled "miosis"). It can be a harmless reflex to bright light or a sign of something that needs quick care. This short guide explains common causes, simple checks you can do, and when to get medical help.

Common causes

Start with the obvious: bright light makes pupils constrict. Beyond that, common reasons include medications and toxins, nervous system problems, and eye conditions.

Medications and drugs: opioids (like morphine, heroin, fentanyl) cause pinpoint pupils. Other drugs that can shrink pupils include pilocarpine (used in glaucoma), some migraine meds, clonidine, and certain insecticides (organophosphates).

Nervous system causes: conditions that affect the pathways controlling pupil size, such as Horner syndrome or brainstem injuries, can lead to lasting miosis. Infections or bleeding in the brain, and some nerve palsies, may also change pupil size.

Eye problems: inflammation inside the eye or prior eye surgery can make one pupil smaller than the other. Sometimes the difference is normal for a person, but sudden changes are worth checking.

Tests, treatment and when to seek help

Do a quick check at home: are both pupils small or just one? Do they react to light? If pupils constrict when you move to bright light and return to normal in dim light, that's usually benign. If one pupil is much smaller than the other, or if you see vision changes, drooping eyelid, severe headache, confusion, or recent head injury, get urgent medical help.

At the clinic, a doctor will check pupil size and light response, ask about medicines and drug use, and may do a basic neurologic exam. Tests can include eye drops to see how the pupil reacts, a urine drug screen, or imaging (CT or MRI) if a brain problem is suspected.

Treatment depends on the cause. For opioid overdose with tiny pupils and poor breathing, naloxone can reverse the effects — call emergency services immediately. If a medication causes chronic miosis, your prescriber may change the drug. Neurologic causes like stroke or hemorrhage need urgent hospital care. Eye inflammation gets treated by an eye specialist.

Prevention and practical tips: wear sunglasses in bright environments to avoid constant constriction, follow medication instructions, and store medicines safely. If you use opioid painkillers, keep them only as directed and ask your doctor about side effects.

If you're unsure, take a photo of the pupils and note when the change started. Sharing that with a clinician helps them decide if you need tests fast. Small pupils are often harmless, but sudden or asymmetric changes deserve prompt attention.