Health experts agree that smoking marijuana can damage the lungs. The deleterious effect of chronic, frequent use of marijuana upon the brain is also well established. However, while more research is required regarding the potential beneficial and/or detrimental effects of marijuana on vision, we do know that the drug has a few key effects on the eyes.

One of the most noticeable effects occurs shortly after smoking the drug: red eyes. THC (tetrahydrocannabinol) in marijuana lowers blood pressure which dilates the blood vessels and increases blood flow throughout the body. This causes the blood vessels in the eyes to expand, causing redness or bloodshot eyes.

Researchers are still examining the potential negative impact of marijuana on peripheral vision, changing eye pressure, and visual processing. Using marijuana may have potentially serious impacts on these areas.

Some people wonder about the potential positive effects of the marijuana for glaucoma patients. Glaucoma is a disease of the optic nerve and patients with glaucoma have increased eye pressure. While marijuana does decrease pressure in the eyes, it does so for a relatively short time. Since marijuana also usually impacts people’s ability to concentrate, make decisions, drive, operate heavy machinery, etc., the drug is not currently a recommended treatment for the disease.

One study of tadpoles suggests marijuana may enhance night vision, while a human study indicates that regular cannabis use may delay the processing of visual information in the retina.

Some studies on glaucoma and marijuana have found that when marijuana is smoked or when a form of its active ingredient is taken as a pill or by injection, it does lower pressure in the eye, or intraocular pressure. However, it only lowers the pressure for a short period of time—about three or four hours.

The study found there was a delayed response time for marijuana users compared with people who did not use the drug. This may impact the eyesight of people who use marijuana regularly, even if the influence is very weak, the researchers said.

In the new study, researchers at McGill University in Montreal applied a synthetic cannabinoid to the eye tissues of tadpoles. They found the cannabinoids made certain retinal cells more sensitive to light, and improved the speed and which the eye responded to even dim stimulus, The Guardian reports. Cannabinoids are the general class of chemicals that can interact with cannabinoid receptors in the brain. The body naturally produces some cannabinoids. Other cannabinoids come from marijuana or other sources.

How Cannabis Affects Our Vision

Many cannabis consumers have experienced the infamous red eye. The eyes are often the most common giveaway after a serious session with a vaporizer.

Yet, while some may find the herb’s visible effects on the eyes to be burdensome, there are several surprising ways that cannabis supports ocular health.

From easing symptoms of eye disease to staving off degenerative blindness, here’s what you need to know about how cannabis affects the eyes:

Already, there are several acute ways that cannabis affects the eyes. While some are spectacular, like improved night vision, not all of the effects are enjoyable. Shortly after consuming the herb, it is not uncommon to experience:

1.) Red Eye

Red eyes are not always the favorite cannabis side effect. While red eyes may be a tell-tale giveaway that you’ve consumed a little cannabis, those who experience tension around the eye might appreciate the relaxing effects of the herb.

Cannabis lowers blood pressure. This causes capillaries and blood vessels to dilate, leading to what is commonly referred to as red eye.

Wider capillaries mean blood flows into the eyes, reducing intraocular pressure. In a way, this a form of relaxation for the eyes. This could be beneficial for patients with painful ocular conditions like glaucoma. 

2.) Allergy

It’s not uncommon for some people to experience an allergic reaction in the eyes after smoking cannabis. This reaction can be triggered by an allergy to smoke, residual molds, or the cannabis plant itself.

Signs of allergy typically include itchiness, redness, inflammation, tearing, and dryness.

In 2015, research from the American College of Allergy, Asthma, and Immunology suggests that cannabis allergy is similar to Hay Fever, causing eye irritation and an itchy nose for those exposed to smoke, pollen, or plant material.

3.) Enhanced night vision

Laboratory research like that mentioned above has found that cannabinoid receptors seem to help the eyes respond to light.

Additional research from 2016 suggests that cannabis compounds, like psychoactive THC, connect with these receptors. Engaging these receptors is thought to improve the ability to see in low-light conditions.

For the past three decades, researchers have speculated that cannabis may also improve night vision. In the 1990s, M.E. West, a pharmacologist, noticed that Jamaican fishermen who consumed a cannabis elixir had an “uncanny ability to see in the dark.”

A small study conducted in 2004 tested the effects of traditional cannabis Kif and a synthetic THC in three Moroccan individuals. Kif is a mixture of cannabis and tobacco. This small experiment found a dose-dependent relationship between cannabis consumption and improved night vision.

4.) Visual processing

Interestingly, recent evidence suggests that the endocannabinoid system contributes to visual development in the brain.

study from University of Waterloo, University of Auckland and Brown University found that babies exposed to cannabis in utero scored significantly higher in visual processing tests.

While babies exposed to alcohol scored lower in visual processing tests, those exposed to cannabis had improved global motion perception.

This is certainly not a reason to consume cannabis during pregnancy, but these findings do add fuel to the idea that cannabinoids and the ECS help the eyes and brain make sense of visual information.