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The pupils are openings that allow light to enter the eyes. The iris––the colored portion of the eye––controls the size of the pupils. The pupils are constantly changing. They enlarge (dilate) in darker conditions to allow more light to enter the eye, and they get smaller (constrict) in brighter conditions to minimize the amount of light that enters the eye.
Some medications and recreational drugs can cause the pupils to dilate, with the pupils returning to normal when the effect wears off.
Immediate medical care, however, should be obtained for pupils that remain dilated, or pupils that are unequal in size, as this can be a sign of a serious condition affecting the brain, including stroke, internal bleeding, or tumor. Similarly, a head trauma that produces dilated or unequal pupils may indicate a more significant problem that requires immediate medical attention.
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