Dec 22, 2019

Updated on Oct 18, 2021

8 min read

Why does cannabis make your eyes red?

Scientifically reviewed by Codi Peterson, PharmD

Red eyes from cannabis are entirely harmless, but here’s how to manage the most obvious sign of being medicated (Shutterstock)

Red eyes are one of the most common side effects of marijuana, and a tell-tale sign that someone in the building likely just got high. Those red, glassy stoner eyes are caused by the fact that cannabis can dilate certain blood vessels and leave you with natural, rose-colored glasses to view the world with.

Why do red “stoner eyes” happen?

Cannabis can have a number of side effects, many of which are related to the cardiovascular system. These side effects are a result of how cannabis interacts with the endocannabinoid system, which helps to regulate a wide range of bodily functions ranging from hunger to sleep to pain to heart rate.

The plant-based cannabinoids in cannabis (“phytocannabinoids”) can trigger the receptors of the Endocannabinoid System, and alter how they control other bodily functions, including the cardiovascular system.

This results in another common side effect of cannabis – an increased heart rate and lower blood pressure. Perhaps most importantly, THC is the chief offender in these cardiac effects, as they are thought to be primarily mediated by activation CB1 receptors.¹

THC can cause the blood vessels to dilate, leading to tachycardia (an increased heart rate) for a few hours after consumption and lower blood pressure, including intraocular pressure. Once more blood is flowing through the capillaries, more blood can also flow around the eyes, causing redness. ²

Does weed affect your pupils?

The red stoner eyes can also become more apparent due to a somewhat common side effect of cannabis – dilated pupils. A 2008 study found that smokers of cannabis in the form of Moroccan “kif” had significantly increased pupil size when high, while a 2018 study examined drivers suspected of driving under the influence and found that 63.6% of subjects who had used THC had dilated pupils. ³ ⁴

This effect has not been consistent across the board in scientific studies, However, even if cannabis does dilate your pupils some, you should not expect to now have night vision or any other super powers to speak of (sorry). ⁵

Does everyone get red eyes when they are high?

Not everyone gets those red, stoned eyes when they consume cannabis, or when they do, it may be less striking than with other people. Additionally, like other effects of cannabis, tolerance can build to some, but unlikely all, of the eye redness.

It’s reasonable to suggest that consuming lower THC cannabis could lessen the red eye effect.

Some people can be more sensitive to the cardiovascular effects of cannabis, or they may be more sensitive to the drying effects of THC like cotton mouth or dry eyes. Of course, many people also have allergies that can irritate or dry their eyes out more and produce a more stark red-eye effect.

How long do red eyes last from weed?

Red eyes can make it difficult to remain subtle about getting high, but there are some very easy fixes (Pexels)

The good news about red stoner eyes is that it is far from permanent,, and is usually an hour or two after getting high.

A 2021 report from the American Academy of Ophthalmology on marijuana and glaucoma put a scientific spin on this question. The report stated that “research in the 1970s and 1980s did show a measurable decrease in intraocular pressure for three or four hours after smoking cannabis or ingesting pure THC as a pill or injection.”⁶

What can we extrapolate from this? That in the first few hours after ingesting cannabis, there can be a decrease of intraocular pressure, increasing the blood flow through the eyes – and possibly making them red.

The red eyes side effect can last as long as someone is under the influence of cannabis, and the intake method used can play a role in this. Edibles have a much longer onset than smoking cannabis, and the effects can be felt for much longer. If you are prone to getting red eyes when you’re high and have ingested a generous dose of edibles, you may have red eyes for six hours or more.

What can you do to avoid red “stoner eyes”?

Red eyes are not unavoidable when consuming cannabis, and even if you’re prone to get red eyes, it can fluctuate and be better or worse at times, due to a number of factors.

Consider a different form of cannabis

If you have found that high-THC cannabis (Type I) makes your eyes more red, or that smoking a blunt or a joint instead of a one-hitter or vape leaves you red-eyed, then consider mixing up your intake method a bit. Again, red eyes is a perfectly harmless side effect, but if you’d rather avoid it, pay attention to which types of cannabis produce the strongest red eye effect, and react accordingly when you prefer to have eyes that don’t broadcast to the world that you just got high.

Stay hydrated

Dry, red eyes can also be the result of dehydration. Staying hydrated is important no matter what you have planned, but if you have a big smoke session coming up later in the evening, consider drinking a lot of water beforehand.

Crack a window

Red eyes are not caused by the smoke of cannabis, but smoke can irritate your eyes, especially a dense cloud in a closed off room. Try not to hotbox the room too much, and get a good breeze going.

What can you do to clear up red eyes?

Visine or Clear Eyes drops are the quickest solution to instantly clear red eyes (Pexels)

Countless students heading back from a break between classes or wedding guests returning from “getting something I forgot in the car” know that there are a few steps you can take to make those red eyes not such a big deal.

Eye drops

The most tried and true method, eye drops can have varied success in clearing up red eyes. Look for over the counter eye drops that are specifically made to treat red eyes. These drops contain vasoconstrictors meant to counteract the dilation effect from THC. Allergy(antihistamine) + redness relief eye drops can be a great option if you’re someone who also has seasonal allergies.

A cold compress

An ice pack or a cold, wet washcloth held over the eyes for a few minutes can help clear up the eyes some. Again the idea is that the cold causes the blood vessels to constrict. It can also just be a refreshing, cool feeling when you’re already a little bit high.

Wear some sunglasses

If you can’t beat them, cover them with some shades. Wearing sunglasses is probably not an option indoors unless you are Bono, but if you’re out and about running errands and you’d like to cover up your red eyes, then shades can get you there even without the use of eye drops.

Can CBD make your eyes red?

We addressed THC already, but what about Cannabidiol (CBD), the highly-popular, non-psychotropic cannabinoid?

Research has shown that CBD can temper the effects of THC, so if you find that high-THC strains make your eyes more likely to go red, then consider using some CBD, or a strain that has a higher CBD to THC ratio (eg. Type II).

A 2017 study found that even just a single dose of CBD can decrease blood pressure, indicating that it is possible that CBD could increase the blood flow to your eyes. At the same time, a rodent trial carried out in 2018 found that while an application of THC ointment reduced IOP in mice, a similar application of CBD “has two opposing effects on IOP and can interfere with the effects of THC,” and prevent it from lowering ocular pressure. ⁷

While it is unclear what role this could have in regards to eye redness, the fact that it appears to increase pressure inside the eyes of mice suggests that “the use of the substance in the treatment of glaucoma may actually worsen the condition.” ⁸

Do edibles make your eyes red?

Spoiler alert: it’s not the smoke that’s making your eyes red. While it’s true that smoke can irritate the eyes, it is the effects of the phytocannabinoids in cannabis that cause red eyes.

This means that you can still get red eyes when you take edibles, especially high THC edibles that produce a strong intoxicating effect. Because the edibles high can last much longer than the high of smoking weed, you’ll want to remember to stay hydrated during the edibles session, and you may want to keep the eye drops within reach.

Is cannabis effective for glaucoma?

Cannabis does help with glaucoma but isn’t the most efficient medicine for the same (Shutterstock)

For countless people, the first medical use of cannabis that they heard of was for the treatment of Glaucoma, an eye condition that is often caused by high pressure in the eye. The pressure can cause serious pain and vision problems, and many patients have found that cannabis can provide relief.

The wide-spread awareness of cannabis as a treatment for Glaucoma can be linked to a 1971 study that found that smoking marijuana could lower IOP, and the struggle of a DC man named Robert C. Randall who in 1976 won a landmark court case to allow him to grow cannabis to treat his glaucoma. The case is credited with helping establish marijuana as an accepted form of medical treatment. ⁹ ¹⁰

But while a 1998 study found that smoking cannabis “causes a fall in intraocular pressure (IOP) in 60% to 65% of users,” the researchers also said that the rate of use needed to maintain this “would lead to substantial systemic toxic effects revealed as pathological changes.”

One researcher estimated that the short term effects would require 8 to 10 joints smoked every 24 hours to maintain the reduced IOP.

Also, researchers have stated that while marijuana can reduce IOP, it can also decrease blood pressure, “which can reduce blood flow to an already compromised optic nerve.”¹¹

Glaucoma patients who are considering the use of cannabis should be advised that there are other drugs that can lower IOP for longer durations, and that any treatment regimen should be devised through the supervision of a qualified physician.

Let your red eyes shine – or don’t

At the end of the day, red eyes fall into the category of “harmless side effects that can make things awkward at work or around your in-laws.” They can make it very difficult to keep your weed intake discreet, and can also lead to some teasing from your friends.

At the same time, the side effect isn’t anything to be afraid of, and when it does happen it’s easily treatable with some tips that are just good to keep in mind in general: find the eye drops that work best for you, stay hydrated, keep a window cracked, and wear shades if you really need to get out there and socialize.

Sources

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Miller, Sally, et al. “Δ9-Tetrahydrocannabinol and Cannabidiol Differentially Regulate Intraocular Pressure.” Investigative Ophthalmology & Visual Science, vol. 59, no. 15, Dec. 2018, pp. 5904–11. iovs.arvojournals.org, https://doi.org/10.1167/iovs.18-24838.
Merzouki, A., et al. “Assessing Changes in Pupillary Size in Rifian Smokers of Kif (Cannabis Sativa L.).” Journal of Forensic and Legal Medicine, vol. 15, no. 5, July 2008, pp. 335–38. PubMed, https://doi.org/10.1016/j.jflm.2007.08.001.
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Brown, B., et al. “Pupil Size after Use of Marijuana and Alcohol.” American Journal of Ophthalmology, vol. 83, no. 3, Mar. 1977, pp. 350–54. PubMed, https://doi.org/10.1016/0002-9394(77)90732-2.
“Does Marijuana Help Treat Glaucoma or Other Eye Conditions?” American Academy of Ophthalmology, 8 Mar. 2021, https://www.aao.org/eye-health/tips-prevention/medical-marijuana-glaucoma-treament.
Jadoon, Khalid A., et al. “A Single Dose of Cannabidiol Reduces Blood Pressure in Healthy Volunteers in a Randomized Crossover Study.” JCI Insight, vol. 2, no. 12, June 2017. insight.jci.org, https://doi.org/10.1172/jci.insight.93760.
“CBD in Marijuana May Worsen Glaucoma, Raise Eye Pressure: Research in Mice Suggests over-the-Counter Substance Could Possess Unknown Side Effects.” ScienceDaily, https://www.sciencedaily.com/releases/2018/12/181217151537.htm. Accessed 4 Oct. 2021.
Hepler, R. S., and I. R. Frank. “Marihuana Smoking and Intraocular Pressure.” JAMA, vol. 217, no. 10, Sept. 1971, p. 1392.
Zielinski, Graeme. “Activist Robert C. Randall Dies.” Washington Post, 8 June 2001. www.washingtonpost.com, https://www.washingtonpost.com/archive/local/2001/06/08/activist-robert-c-randall-dies/c6e832a4-55e2-47fc-a3c8-5e011da66e04/.
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