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Understanding Cannabis & Red Eye

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Cannabis & Red Eye

Why do hemp and cannabis consumers sometimes suffer red eye? Below, students learn the science-based research regarding the biochemical mechanisms of legal marijuana and the human eye—including red eye, dry eye, night vision, and overall retinal health.


Common sense might convince one that any eye irritation suffered by hemp consumers, including excessive redness, results from literal smoke in their eyes. However, scientists have discovered some of the ocular biochemical mechanisms involved in the consumption of cannabis by humans, one of which is why users sometimes suffer red eye.

A collection of popular eye drop brands.
Does cannabis cause red eye?

Biochemistry of Cannabis & Red Eye

Modern research has taught us that the red eye associated with cannabis smokers for decades (which has spurred thousands of social media memes) results not from smoking, but rather the consumption of marijuana—more specifically—delta-9 tetrahydrocannabinol (THC).


Those who use relatively large quantities of THC via other consumption avenues, such as eating or sublingual tinctures that deliver this cannabinoid directly to the blood stream, also can suffer from red eye. Your takeaway, students: Red eye in cannabis consumers is not a smoke thing.


"When blood pressure begins to decrease, it causes the dilation of capillaries and blood vessels in the eye."

Most cultivars ("strains") of cannabis result in increases in heart rate and blood pressure. This increase, however, is of short duration, lasting only five to 10 minutes. When blood pressure begins to decrease, it causes the dilation of capillaries and blood vessels in the eye. This dilation results in increased blood flow. It is this increased blood flow that causes the effect of red eye (it also results in a reduction in intraocular pressure, which is why cannabis is helpful for glaucoma patients).


"It's cannabis' ability to reduce intraocular pressure in the eyes that makes it a potentially viable treatment for glaucoma...which can eventually lead to blindness," said Dr. Melanie Bone, a doctor based in West Palm Beach, Florida. "It also happens to explain why your eyes become bloodshot after smoking cannabis," she added.

A closeup shot of a woman's eye.
Cannabis causes increased blood flow.

Cannabis & Red Eye Studies

Cannabis & Red Eye. Research has revealed that cannabis and THC sometimes produce not only the red eye. Marijuana consumers sometimes suffer many other ocular characteristics—including tearing and "dry eye," light sensitivity and night vision acuity, and overall retinal health.


This study cited previous reports that documented an improvement in night vision "among Jamaican fishermen after ingestion of a crude tincture of herbal cannabis."

Cannabis & Red Eye: 2022 Study

A 2022 study entitled "Sex Differences in Medical Cannabis-related Adverse Effects? Do Not Forget the Context!" that was published in the journal Pain reported that red eye is one of the only adverse effects of cannabis that occurs more frequently in men than women. The same study reported that a related but technically different adverse effect of marijuana, dry eyes, occurs more in women than men.

A closeup photo of a woman's eye.
Much research to date.

Cannabis & Red Eye: 2020 Study

A 2020 study entitled "THC Regulates Tearing via Cannabinoid CB1 Receptors" that was published in the journal Ophthalmology and Visual Science and conducted on rodent subjects investigated how cannabis and THC might effect the tearing mechanisms of the eye that prevent it from becoming too dry.


"Cannabis users frequently report dry eye, but the basis for this is unknown," reported the study's authors. "If the effects occur via the endogenous cannabinoid signaling system [endocannabinoid system, or ECS]...this may represent a novel mechanism for the regulation of tearing."


"The study found the presence of CB1 receptors in the lacrimal gland of the eye and identified a gender-dependent response curve."

The study found the presence of CB1 receptors in the lacrimal gland of the eye and identified a gender-dependent response curve. "Activation of CB1 receptors by THC...reduces tearing in male mice. In female mice, THC has no effect."


The researchers concluded that a physiological mechanism is responsible for "dry eye symptoms," reporting that stimulation (binding) of the "neuronal CB1 receptors in the lacrimal gland reduces tearing."

Two fan leaves from a young cannabis plant.
Red eye depends on cultivar.

Cannabis & Red Eye: 2016 Study

A 2016 study entitled "Cannabinoid Receptors CB1 and CB2 Modulate the Electroretinographic Waves in Vervet Monkeys" that was published in the journal Neural Plasticity explored "the role of cannabinoid receptors CB1 and CB2 in the normal monkey retina."


The researchers found an "abundance" of CB1 and CB2 receptors in the retinae of the test subjects and cited the "important role of these receptors in normal vision." The study's authors concluded that "manipulating the endocannabinoid system might...serve as a therapy to restore normal vision and protect the retina."


The researchers found an "abundance" of CB1 and CB2 receptors in the retinae of the test subjects and cited the "important role of these receptors in normal vision."

Cannabis & Red Eye: 2009 Study

A 2009 study entitled "Endocannabinoids in the Retina: From Marijuana to Neuroprotection" that was published in the journal Progress in Retinal and Eye Research investigated the biochemistry of the body's internally produced cannabinoids that are called endocannabinoids (versus marijuana-based phytocannabinoids).


The study confirmed other research by finding that "smoking marijuana induces corneal vasodilation (so called red eye) and a reduction of intraocular pressure." It dispelled the myth that cannabis dilates the pupils of users. "The popular idea that marijuana dilates the pupil has not been supported by experimental data," reported the scientists. This study also confirmed the mechanism of THC molecules binding with CB1 receptors within the eye.


The study confirmed the common belief that cannabis consumers are more sensitive to light. "Acute effects of marijuana smoking on vision include...alterations in color discrimination and in increase in photosensitivity."

A man touches a cannabis plant in a commercial garden.
More research is needed.

Cannabis & Red Eye: 2004 Study

A 2004 study entitled "Cannabis Improves Night Vision: A Case Study of Dark Adaptometry and Scotopic Sensitivity in Kif Smokers of the Rif Mountains of Northern Morocco" that was published in the Journal of Ethnopharmacology explored the decades-old urban legend that cannabis use improves night vision.


The study's authors reported that "improvements in night vision measures were noted after THC or cannabis."

This double-blind placebo-controlled human trial study cited previous reports that documented an improvement in night vision "among Jamaican fishermen after ingestion of a crude tincture of herbal cannabis" and noted that "Moroccan fishermen and mountain dwellers observe an analogous improvement after smoking kif, sifted cannabis sativa mixed with tobacco."


The study's authors reported that "improvements in night vision measures were noted after THC or cannabis." The researchers clarified by reporting that this mechanism, like many others involving cannabinoids and terpenes, is dose-dependent. They concluded that further testing is necessary to "assess possible clinical applications of these results in retinitis pigmentosa or other conditions."

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