How does marijuana affect vision?

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.

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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.

How Does Cannabis Affect the Eyes?

As it turns out, cannabis may help you see things a little differently. Research suggests that the herb can have an impact on every organ in the body, including the eyes.

While it may sound too good to be true, cannabis compounds work their magic in the eyes by tapping into one of the largest cellular communication networks in vertebrates. This network is the endocannabinoid system (ECS).

Cannabis compounds interact with the ECS by engaging a special type of cell receptor, called a cannabinoid receptor.

The human eye happens to express high levels of one particular cannabinoid receptor, the CB1. Vision processing centers of the brain also feature a bounty of these landing sites.

In addition, preclinical investigations suggest that the ECS plays a crucial role in our vision.

A 2016 primate study published in Neural Plasticity has found that manipulating cannabinoid receptors changes the way electroretinographic waves pass through the retina.

These waves are measured by an electroretinogram, which records the electrical response of the eye to a light stimulus. The researchers found that cannabinoid receptors moderated the eye’s response to light. 

The ways in which cannabis affects eyesight needs further investigation. However, this early research offers even more reason to investigate cannabis as a treatment for diseases of the eye.

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:

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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.

A 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.

Do edibles make your eyes red?

Similar to smoking cannabis, ingesting edibles could also make your eyes turn red. Again, this depends on the amount of THC consumed. Remember, it’s not the smoke itself that makes your eyes red, but rather the ability that cannabinoids have to lower blood pressure, causing blood vessels and capillaries to dilate. 

The redder the better?

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The amount your blood pressure is lowered and how red your eyes become depends on the amount of THC you consume.

Tetrahydrocannabinol (THC), the most common cannabinoid in the plant, is responsible for the intoxication associated with smoking cannabis. The greater the concentration of THC in a cannabis product, the stronger the effects and the redder your eyes become.

New Research Suggests Marijuana May Improve Vision

The first studies on how cannabis affects vision seemed to demonstrate that those effects were either non-existent or negative. This research proposed that cannabis causes a delay in the processing of visual information in the brain, so that there could be a lag time between seeing something and taking action in response to it, such as making decisions while driving, moving or picking up objects. Other research based on interviewing cannabis consumers about their vision revealed that some people felt that they saw better while taking cannabis, others felt their vision was worse, and some didn’t report any change at all.

But those differences could be explained by factors including the kind of cannabis a person used, how often they used it and under what circumstances—and even the amount of CB1 receptors an individual has in given areas of the body. What’s more, the activity of those receptors can be affected by numerous factors such as the number of natural cannabinoids in a person’s system or other health conditions that affect the ECS in general. Those variations mean that cannabis can have a range of different effects on different people.

Now, though, recent research that takes into account new discoveries about the ECS reveals a different and far more positive picture. The action of cannabinoids on CB1 receptors in the eye, particularly the retina, seems to increase visual sensitivity in low light and may also increase visual contrast—the ability to pick out things from their background. Though the research involved tadpoles, not humans, these insights could have implications for people whose work requires good night vision, or for those who have problems with distinguishing between things of similar color or brightness.

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