How To

What Causes Your Eyes to Dilate?

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The mirror tells a story as you look into it, one you know everyone is going to read the second you step outside. They’ll look into your eyes and not because they’re searching for anything, but because that’s where our minds can be glimpsed, if only for a moment.

If only you had some sunglasses or some eye drops. You can feel panic starting to set in. Did you make a horrible decision? Oh god this is going to be bad. There has to be some way to stop your eyes from looking like this!

How are you going to face your friends, or what if your teacher notices and calls your mom?!

Dilated pupils can certainly bring a lot of unintended consequences along with them, including ideas other people have about what they mean. Sometimes they’re easy to spot, too.

We look people in the eye out of a natural desire to know them, to listen, and to experience their lives. Whether we’re consciously aware of it or not, when someone’s eyes are dilated we may take notice.

Your eyes are always dilating and constricting (getting smaller) all throughout the day. It’s a natural thing, something your body needs to stay healthy in a number of ways.

There are a lot of reasons why a person’s eyes will dilate, such as being happy, walking into a room and turning the light off or, potentially, even talking to someone you like or love.

Specifically though, why do certain drugs and substances cause eyes to dilate? Maybe you’ve heard someone say “Did you see that person’s eyes? They were definitely tripping,” or even “That person is majorly high right now. Their pupils were massive!”

A lot of people have questions about why specific drugs or substances cause eyes to dilate, so in order to better understand that let’s first look at how eyes dilate in general.

dilated pupils

How Pupils Dilate and the Cause

The scientific term for pupils dilating is mydriasis. When a pupil gets smaller it is called miosis. Both can happen in response to certain drugs and substances entering a body, but let’s focus on dilating for now.

According to a study by Sebastiaan Mathôt about how our eyes work, pupils dilate in response to three things: when the level of light changes, when we need to focus on an object that is closer to us, and when our bodies are responding to either outside or internal stimulus.

One thing Mathôt specifies that may seem strange is that our pupils will only constrict, meaning get smaller, when responding to light. This means when your pupils are getting wider due to light it will only be because they have already constricted.

And why do our pupils get smaller in response to light? It’s because our eyes are controlling the amount of light entering our eyes, protecting us from receiving too much.

Similarly, our pupils will get smaller to help us see things that are closer to us. They’re somewhat like a camera lens (just far more complex, obviously) so the function of getting smaller is almost like your eye is zooming in. That is an incredibly simplified version of how it works, however.

The third reason for dilation, a response to an outside or internal stimulus, is where our interest lies here. The external stimulator of a drug or substance affects how our eyes behave, so let’s look at how that happens.

Why Taking Drugs Causes Dilated Pupils

Your body’s method for constricting your pupils is a muscle around your eyeball called the iris sphincter. It’s controlled by your parasympathetic nervous system, which is in charge of making sure your body stays in “stable condition.”

You can maybe already see how some related drugs affect eye pupil size when you know your eyes’ dilation is controlled by your nervous system.

When a drug or substance that affects your nervous system is introduced into your body it has a chance of affecting your eyes. It won’t always, depending on the substance.

The most common drugs that can cause eyes to dilate are:

  • Cocaine
  • Methamphetamine
  • LSD
  • Marijuana

All of these drugs will have some sort of effect on your nervous system, but in the case of cocaine, your pupils dilate due to your brain’s reaction to norepinephrine.

For methamphetamines your body is reacting to dopamine, also called the “reward” chemical in your brain. As meth enters your system your brain releases dopamine which triggers a “reward” response in you, including dilated pupils.

LSD causes “hyperarousal of the nervous system,” which of course then causes the pupils to dilate since they’re controlled by your nervous system.

It’s a little more complicated for marijuana. Studies don’t show a lot of data to indicate someone smoking or ingesting marijuana will definitely have dilated pupils. It can happen, but really, there’s no definitive answer for how or why.

Sometimes it’s assumed someone’s pupils dilating after smoking marijuana may be due to other factors, but also, our bodies have enough differences that it is hard to pin down.

Pupil Constriction

When your pupils get smaller it’s called miosis. A variety of drugs can cause this, some prescription and some not. The most common type of drugs that cause miosis are opiates.

There are some other interesting facts about pupil constriction in relation to opiates like heroin and morphine. Pupils will typically constrict within 15 minutes to an hour of heroin entering the body and can last between three and five hours.

Studies have been conducted on how long the pupils will stay constricted in patients who are dependent on heroin and those who aren’t. Someone who is non-dependent can experience constricted pupils for up to two hours, while someone who is dependent will have their eyes return to normal within 15 minutes.

Can You Tell What Drugs Someone Is Taking By Checking Their Eyes?

The simplest answer is no, you can’t. You can make an educated guess or even an uneducated guess, but in order to know for sure, you will need a lot more information.

Remember, all of our bodies are reacting to a lot of different factors all at once. There are tests that utilize pupil size as one piece of information in determining an answer to what substance someone may be on, but using it alone won’t indicate, without doubt, why someone’s eyes are dilated or constricted.

Most of the tests conducted in treatment facilities or by police are relying on multiple steps like checking blood pressure, pulse, and temperature.

There is generally no one piece of information that is going to accurately predict what drug someone has ingested.

Usually, people respond to dilated pupils by wearing sunglasses to hide them or to protect their eyes from the light, which feels much brighter than usual. Eyes can also become red (sometimes permanently) from prolonged marijuana use. Eye drops are sometimes used to try and fix this.

If someone is trying to hide their drug or substance use they may be struggling with addiction. It’s not always the case, obviously, but the potential for a problem is concerning enough it can be important to talk to them and check-in.

They may need help.

SUN Behavioral Can Help

If you or a loved one is struggling with drug or alcohol use, we’re here for you. If there is also a struggle with mental health SUN Behavioral has co-occurring treatment options that can help address every aspect of your struggle. Call us at (859) 429-5188 to ask about your treatment options. We want to help.

Frequently Asked Questions

What Does It Mean When Your Pupils Are Dilated? What changes occur?

It can mean many things. Your eyes may be reacting to light by contracting (getting smaller) or because they are trying to focus on an object that is close to you. Another possibility is that you have seen something that excites you, which would be called an “external stimuli.” There are lots of studies that focus on how our eyes react (or if they do) when seeing someone we find attractive, or even just seeing an object we like. Science is kind of divided on whether your eyes dilate when you are in love with someone and whether it’s a common reaction. The other possibility is that you have ingested a drug or substance that caused your eyes to dilate. Different substances have varied effects on your pupils, but some drugs like hallucinogens and cocaine can make your pupils dilate drastically.

Can Stress Cause Your Pupils to Dilate?

Potentially, although there are not a lot of definitive studies on this. Because general understanding of the actual reason (not the mechanism) for our eyes dilating in certain situations is unknown, there has been some information that shows your pupils can dilate due to stress (this is what’s called “anecdotal evidence”). Some doctors have seen stress lead to dry eyes, twitching eyelids, and occasionally yes, dilated pupils, but there is more evidence pointing to those situations being rare. More common, regarding stress and dilated pupils, is if someone is experiencing shock. A person in shock will have other symptoms such as chest pain, labored breathing, severe pain, and can also include slow or weak pulse, nausea, shaking/shivering, and dilated pupils.

What Drugs Cause Dilated Pupils?

The most common drugs that can cause dilated pupils are: cocaine, methamphetamine, LSD, and marijuana. Other drugs can cause your eyes to constrict (called miosis); the most common one with this effect is heroin. Alcohol, barbiturates, and heroin can cause diplopia, which is the scientific term for “seeing double.”

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