Medically Reviewed by
Jason Crawford

Article Last Updated on January 15, 2023

Cannabis is a very special plant with unique properties and chemical composition which sets it apart from many other plants. It produces a variety of effects that can be used for both medicinal and recreational purposes, making it very versatile.

However, although legalized and/or decriminalized in most states, weed is still classified as a drug due to its psychotropic properties. This also means that weed is capable of inducing some side effects that are harmless most of the time but can feel weird.

A lot of other psychotropic drugs cause visible signs of being under the influence shortly after use, such as pupil dilation, so a common question is whether smoking weed also affects pupils and what actually happens to your pupils when high.

In today’s article, we’ll discuss why pupils dilate and whether marijuana can cause changes to pupil size, as well as other visible signs that come with smoking weed.

Why Does Pupil Dilation Happen?

The most common reason why pupils dilate is when the eyes are exposed to low lighting conditions, so the pupils dilate to let more light in to improve vision. However, many other things can cause pupil dilation, such as looking at something you like, eye injuries, as well as certain medications, and drug use.

Pupil dilation, also known as mydriasis, occurs when the iris sphincter and the iris dilator muscles get activated. This reaction is the result of two different responses by two divisions of the autonomic nervous system. 

The parasympathetic nervous system, which regulates unconscious bodily functions (so-called “rest-and-digest”), triggers the iris sphincter muscles and causes the pupils to constrict. Contrary to that, the sympathetic nervous system, which maintains homeostasis and regulates the fight-or-flight response, triggers the iris dilator muscles and causes the pupils to dilate.

When, for some reason, the part of the brain responsible for the fight-or-flight response gets stimulated, it activates the sympathetic nervous system which results in dilation of the pupils.

What Drugs Can Cause Dilated Pupils?

Many psychotropic drugs can stimulate both of these divisions of the autonomic nervous system and cause a different response. So, depending on which drug activates which part of the nervous system, pupil size will also be affected.

For example, hallucinogens like LSD, mescaline, ecstasy, MDMA, psilocybin, amphetamines, and some antidepressants increase the levels of serotonin in the brain (the neurotransmitter responsible for mood). From here, serotonin stimulates the receptors in the brain that are responsible for activating the pupil dilation response. So, does weed dilate pupils?

What Happens When You Smoke Weed

Smoking weed is one of the fastest ways to get high. This is because the smoke you inhale goes straight into your lungs and from there, into your bloodstream and your brain. As soon as you inhale weed, your heart rate increases and your blood pressure temporarily rises. This process takes approximately 20 minutes before you notice your eyes getting glazed, and then you start feeling high.

When weed enters your system, THC (tetrahydrocannabinol) and CBD (cannabidiol) start interacting with cannabinoid receptors naturally present in the body. These receptors form a large neurotransmitter network called “the endocannabinoid system.”

The receptors are present in various locations throughout the body, including the brain, which is how weed is able to produce mind-altering effects. 

The stronger the strain is (meaning the higher the THC content is), the more pronounced the effects will be. The euphoria linked to marijuana that every weed user knows and loves is caused by the increased release of dopamine. In fact, THC stimulates the part of the brain that comprises the so-called “reward system” which releases dopamine under the influence of THC.

Does Smoking Weed Cause Pupil Dilation?

There is conflicting evidence as to whether smoking weed dilates pupils. Even though the release of dopamine seems to influence pupil size, some clinical and anecdotal evidence points towards both ends.

According to one study, THC concentration was linked to pupil dilation, while according to another study, weed caused a constriction (shrinking) of the pupils. That said, the impact of marijuana has so many variables, like individual tolerance and response to the cannabinoids, as well as the effects of different strains. Therefore, it could be concluded that marijuana can indeed change pupil size, but it will vary depending on the situation.

However, pupil dilation is hardly a tell-tale sign that you’ve been smoking marijuana, as most commonly, weed affects the eyes in a slightly different and more pronounced way than pupil dilation.

How Else Does Smoking Weed Affect the Eyes?

The most obvious sign you’ve been smoking weed will be red eyes, hands down. If you’ve been a weed user for a while, you’ll know that it’s one of the more bothersome side effects of cannabis.

The main offender here is THC, the psychoactive cannabinoid. When you smoke weed, THC temporarily raises your heart rate and blood pressure, which lasts about 10 minutes. Afterward, as your heart rate normalizes and your blood pressure begins to decrease, the blood vessels and capillaries also relax, which allows for blood flow to rush into them, including the eye area. Since the eyes are sensitive, they will quickly become red.

It’s not all bad, though. Weed can reduce inner-eye pressure, so patients with glaucoma are often recommended to use weed to relieve their symptoms.

Bottom Line – Dilated Pupils Aren’t the Primary Sign of Marijuana Use

Many psychotropic drugs are known to cause visible signs of being under the influence, pupil dilation being the most common one, so many wonder if marijuana also dilates your pupils. Truth is, there isn’t a straightforward answer to this as it will all depend on how your body handles marijuana, as well as the frequency and strain you use.

However, there is one tell-tale sign that you’ve been smoking marijuana, and that is getting red eyes. The main reason for this is the blood pressure changes that happen due to THC entering your system, which results in a quick rush of blood throughout the body (including the eyes, which are very sensitive). Therefore, getting bloodshot eyes is a dead giveaway you’ve enjoyed a doobie, and it’s far more noticeable than having dilated pupils.

A passionate advocate for the benefits of cannabis. Fraser Horton, who has a background in botany and a strong love of nature, has spent years researching how cannabis affects the body and mind. He established Leaf Nation in 2020, where he has devoted himself to educating people about the legalisation of marijuana and its safe and responsible use. Fraser is committed to highlighting cannabis’ potential for improving wellness and working to dispel the stigma associated with its use.


The information presented on this page is provided as a public service to aid in education and is derived from sources believed to be reliable. Readers are responsible for making their own assessment of the topics discussed here. In no event shall Leaf Nation be held reliable for any injury, loss or damage that could happen if using or abusing drugs.