Miosis is a condition in which the pupil, the black opening at the center of your eye, shrinks and becomes very small. The word is derived from ancient Greek and literally means "to close the eyes."

Your pupils naturally change size thousands of times a day, expanding in dim light and contracting in bright light — this regulates the amount of light that reaches your retina. Pupils also constrict whenever you’re focusing on an object that is very close. 

However, if you experience persistent pinpoint pupils (also known as miotic pupils) in dim light, it is possible you have abnormal miosis. This type of miosis can occur in one or both eyes, and can have a number of different causes, including certain medications, head injury or even the aging process. 

Normal pupils range from 2-5 mm in diameter. In miosis pupils are less than 2 millimeters in diameter in normal light. That is very tiny — a dime, by contrast, is almost nine times larger.

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Pupil constriction due to aging

For the first two weeks of a baby’s life, their pupils will be small, providing protection from bright light. This is normal. 

When people age, the eye muscles that enlarge or shrink the pupil grow weaker. 

As a result, pupils can become smaller and take longer to dilate in the dark. This is called senior miosis and usually begins when a person is over 70 years old.

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Miotic pupils can be caused by medication

Certain medications can cause the pupils to constrict and become small. These include prescription narcotics that are used to control pain, such as codeine, and prescription medicine for health conditions (anxiety, high blood pressure, etc.). 

Substances that might be responsible for your constricted pupils include:

• Opioids for pain management (oxycodone, morphine, hydrocodone and codeine, etc.) that are prescribed by your doctor. Illegal opiates, including heroin, can cause pupillary miosis as well. 

• Medications that are used to treat high blood pressure, such as clonidine.  

• Anti-anxiety sedatives, such as diazepam (also known as Valium). 

• Antihistamines, such as diphenhydramine (commonly found in Benadryl). 

• Medications for Alzheimer’s disease, such as tacrine. 

Other medications that can lead to pupillary constriction include antipsychotic drugs and certain chemotherapy drugs. 

Check with your doctor if you are taking a new medication and notice that your pupils seem smaller.