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How do your pupils work and why do some substances make them dilate?

Written by: Jill Teige
Hosted by: Hank Green

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[ ♪ Intro ].

Whether or not eyes are the windows to the soul, scientifically, pupils are the windows of the eyes. And sometimes those windows are more open than usual, like when you’ve taken a medication that causes pupil dilation.

Or when people use certain recreational drugs. It all comes down to how different chemicals can cause changes throughout your whole body, which can affect little muscles in your eyes. The pupils are openings that let light into the rest of your eye.

And the colored part of your eye, the iris, controls how much light makes it in. The balance of two muscle groups in the iris changes how big your pupils are, like a camera aperture. The iris dilator is arranged like bicycle spokes around the center of your eye.

And when it contracts, it pulls your pupils open wider. The iris sphincter, on the other hand, is in a ring around your pupil that acts like most of the sphincter muscles in your body. It constricts your pupils down to little dots.

So now you know that you’ve got a sphincter in your eye. That’s just a fact for you. Anything that blocks or stimulates either muscle can knock that balance out of whack and cause some really noticeable changes.

For example, drugs that block receptors for the neurotransmitter acetylcholine can cause impressive dilation. These medications are designed to inhibit the parasympathetic nervous system, which controls the involuntary muscle contractions that keep things moving in your body, like your lungs, or the plumbing that gets rid of waste. This system also controls the iris sphincter muscle.

So when it’s inhibited, the iris sphincter can’t counteract the pull from the iris dilator, and you get wide open, "blown out" pupils. Some drugs, like ones for overactive bladders or excessive sweating, target the entire parasympathetic nervous system. They often have dilated pupils or blurry vision as side effects, but those aren’t the goal.

Others, like the drops used to dilate pupils for eye exams, are more targeted and don’t throw your whole body off. Now, other drugs can stimulate the sympathetic nervous system, responsible for your body’s fight or flight response. And these too can cause dilation.

Take adrenaline, which floods your bloodstream when you’re scared but is also a medication for extreme allergic reactions—it’s what’s in the EpiPen. Adrenaline can bind to nerve receptors connected to the iris dilator, making it contract and pull your pupils open wide. If people use drugs like cocaine, amphetamines, or even weed, their brains have more dopamine and norepinephrine floating around.

Usually, either extra neurotransmitters get released or receptors that naturally suck them away get blocked by those drugs. These chemicals are involved in the “high” feeling but also cause dilated pupils. Scientists think that dopamine makes certain nerve cells release more norepinephrine, which works like adrenaline to make the iris dilator contract.

So wide pupils can be caused by a lot of things. But they’re a hint that certain chemicals, illicit or not, are bubbling through your body. Thanks to our Patreon patrons for asking us great questions like this!

If you want to support us and send us questions, and get some other cool stuff you can go to And if you want to learn more about human eyes, check out our video where I explain what those pesky little eye floaters actually are. [ ♪ Outro ].