why does heroin make my pupils smaller

What do pupils do?

The purpose of our pupils is to improve our vision. Pupils allow light to pass through to the receptors so that we can see things. However, the amount of light that is necessary to see something may change depending on the light source. If the source is too bright,  the sphincter muscles of the eye constrict in order to reduce the amount of light that enters the eye. Pupil constriction basically allows you to focus without damaging the eye. In dim light our pupils do the reverse and get larger in order to let more light in.

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How pupils get small and large

heroin makes your pupils smallerPupil size is controlled by two different sets of muscles, the dilator and sphincter muscles.  The dilator muscles are straight muscles, and the sphincter muscles are circular muscles.  These two muscle groups work antagonistically toward each other.  So when our dilator muscles are dominating, they contract, and our pupils get larger.  When our sphincter muscles are dominating, they constrict, and our pupils get smaller.


We know that pupils always get smaller after heroin consumption, and they stay small, even in the dark.  Scientific studies show that tolerance never develops to this side effect.  Therefore, every time a person uses heroin his or her pupils will get smaller.



Why do pupils get small after using heroin?

The reason why pupils get smaller following heroin consumption is because heroin stimulates the Edinger-Westphal nucleus within the brain stem. When the Edinger-Westphal nucleus is activated it sends a signal to the sphincter muscles in both eyes, which causes the sphincter muscles to constrict. We already know that constricted sphincter muscles make pupils smaller. This is the exact reason why pupils get smaller after using heroin.





Miosis vs Mydriasis

The normal size of our pupils vary from 2.5 – 4mm depending upon the amount of light in our environment. When our pupils are less than 2mm in diameter, like when a person uses heroin, it is called “miosis.” When our pupils are greater than 7mm in diameter, such as during heroin withdrawal, it’s called “mydriasis.”


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Pupillary light reflex

The light reflex controls how much light enters the eye. When light is shone into one eye, that pupil constricts but the pupil in the other eye also constricts. If you look closely into someone’s eyes you’ll notice that both the left and right pupil are the exact same size. In fact, if eyes have pupils of unequal size it may indicate a serious health problem, such as a stroke, brain hemorrhage, infection or tumor.


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