Does Weed Make Your Pupils Dilate?

Sophia Delphi May 14, 2022 - 7 min read
Fact Checked
Illustration for Does Weed make dilated pupils

If you were a typical teenager, you probably hated coming home after you’d been out “having a good time” with friends. You’d have to make it through the front door and slip into your room before you’d run into your parents. Otherwise, you’d have to deal with the dreaded eye exam.

“What have you been doing? Your eyes are red and your pupils are dilated! You were smoking marijuana again! You…are…grounded!”

The eyes don’t lie; you were probably doing something they didn’t want you to do.

But they’re lucky they didn’t have to make that claim in court. Red eyes might have been pretty good circumstantial evidence that you were guilty as charged, but dilated pupils wouldn’t have proved that you were smoking weed. According to some research, dilated pupils could have proved that you hadn’t been smoking up.

At this point, it’s far too late to convince your parents to unground you. But here are the facts they didn’t know.

The Effects Of Weed On The Body

Your eyes don’t immediately “change color,” and nothing happens to your pupils after you’ve taken a toke or two. Bloodshot eyes, for example, are only secondary effects of weed’s effects on the body. To understand what really happens, we have to dig a little deeper.

There’s an extensive signaling system in the body, known as the endocannabinoid system (ECS). In fact, all mammals have an ECS.

The ECS plays an important role in an enormous range of bodily functions, including mood, memory, pain control, movement, sleep, and metabolic balance. To do that work, ECS receptors located throughout the entire body exchange messages by means of chemical neurotransmitters. So anything that interacts with the ECS may cause major physiological, psychological, and emotional effects.

The THC and CBD in weed interact with the ECS. Its molecular structure is almost identical to the structure of an important neurotransmitter — so once you have cannabis in your system, the cannabinoids are able to interact with ECS receptors and send “rogue messages” to the brain and body. That’s why weed has the ability to make you high, and why it might lock you to the couch.

Cannabis has other effects as well; the one we need to talk about is blood pressure.

Weed & Blood Pressure

Let’s walk through this step by step. It won’t take long.

Most regular weed users experience a drop in blood pressure after THC and CBD interact with their body’s ECS receptors. Their blood pressure may initially increase, but just briefly. Then it decreases.

If you’ve never thought about what blood pressure actually means, here’s a crash course. Every time the heart beats, it sends a gush of blood pumping through the arteries. The force of the blood pushes against the walls of the arteries. When a doctor or nurse wraps that cuff around your arm they’re measuring the pressure of blood pushing against your artery walls.

What causes blood pressure to drop? It’s usually because the arteries have become wider; when there’s more room for blood to flow, it’s not pushed forcefully against artery walls. That widening of arteries is called vasodilation; it happens, for instance, when you exercise or drink alcohol.

Weed is a vasodilator as well. Your blood pressure falls because the arteries — and other blood vessels in your body — expand.

And there are blood vessels in the eyes.

Weed & Red Eyes

Anyone who’s been around weed smokers knows the telltale effect many exhibits after a smoke sesh: their eyes look bloodshot.

You can now understand why. There are a number of small blood vessels, known as ocular capillaries, in each eye. When THC and CBD interact with ECS receptors near the smoker’s eyes, those capillaries become wider. In turn, more blood flows through them.

Here’s the missing part of the puzzle. The ocular capillaries are located right behind the whites of the eyes — so the increased blood flow is easily visible, making the eyes look red. And in most cases, the more potent the THC, the longer the red eyes will last.

It doesn’t happen to everyone. Some people with a high THC tolerance don’t experience lower blood pressure or vasodilation when they smoke, so their eyes don’t get bloodshot. Those with existing high blood pressure issues may not experience enough vasodilation to make their eyes look red at all.

But vasodilation is why medical marijuana is so often recommended to glaucoma patients. Weed causes the eye’s blood pressure (intraocular pressure) to drop — and that relieves much of the pain that glaucoma causes.

Does Weed Cause Dilated Pupils?

image of an eye zoomed in with leaves in front

All of the effects we’ve discussed so far have been confirmed by research, even if the scientists aren’t sure of all the mechanisms that cause them.

Whether your pupils (the round openings in the middle of the eye) dilate when you smoke weed, though, is still up for debate.

First, let’s define dilated pupils, so we’re all on the same page. Just as vasodilation means that blood vessels get wider, dilated pupils are larger than usual. That’s the reason they dilate your pupils for an eye exam; when the opening becomes wider, the doctor has a better view of the retina and optic nerve deep in the eye.

Since capillaries dilate when you smoke cannabis, it would make sense that pupils might dilate as well. Add another scientific finding to the mix: dopamine — the “feel-good hormone” that floods the body when you smoke up — also causes pupils to dilate.

Case closed, right? Your pupils must dilate when you smoke weed.

Not so fast. Research has reported findings at both ends of the spectrum.

One study looked at police reports for 589 drivers stopped for suspected driving under the influence; the result showed that THC blood concentration correlated with dilated pupils. Another smaller study, though, examined participants after they’d consumed cannabis, and found the exact opposite effect. Their pupils actually constricted (got smaller) instead of becoming dilated (larger).

What can we conclude from such different results? The same thing we conclude from reading many scientific reports: some people respond to stimuli differently than others. The majority of people get bloodshot eyes after smoking weed — but some don’t. Many people’s pupils dilate — but it doesn’t happen to everyone, and some peoples’ pupils may actually constrict.

For that matter, there are a small number of people who don’t get high from smoking weed. To us, that would be a much bigger issue to worry about than bloodshot eyes or dilated pupils.

Do Your Pupils Dilate When You Smoke Weed? FAQ

Q: Are there substances that always cause pupils to dilate?
A: As we’ve seen, “always” is a dangerous word to use. But coke and meth are known to cause dilated pupils in most users. On the other hand, heroin and a number of other opiates usually cause pupils to constrict.

Q: What about alcohol?
A: Your pupils respond in a different way when you drink. Alcohol use slows brain and muscle function and communication. As a result, the pupils don’t dilate or constrict normally when they’re exposed to light; the brain and eyes’ response times are much slower than usual. That’s one of the major reasons why drunk driving is such a huge risk.


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