How Does Marijuana Affect Vision?

Marijuana and the Human Endocannabinoid System

The human endocannabinoid system is a neural network that regulates and influences important functions such as appetite, sleep, pain, immune responses, and brain activity. The chemical compounds in marijuana, called cannabinoids, can interact with this system by binding with CB1 and CB2 receptors – which are the biological equivalent of a control panel for the endocannabinoid system.

CB2 receptors are mostly scattered throughout the body, while CB1 receptors are primarily concentrated in regions of the brain such as the temporal lobe, cerebellum, and neocortex. These regions handle learning, communication, memory, emotions, and movement. They also process sensory inputs like sights and sounds, which gives us a clue as to how marijuana may influence and alter vision.

When THC targets CB1 receptors in the neocortex and occipital lobes, the result may be alterations in the ways sights and sounds are perceived and processed. Both academic and popular literature is filled with descriptions of marijuana causing colors to seem more intense, objects to come into sharper focus, and shapes to appear distorted or change shape and size.

In addition to these perception-altering effects, there is some evidence that marijuana may also influence the mechanics of sight. Recent animal-based research indicates that cannabinoids may also interact with CB2 receptors in the retina, creating changes in visual acuity and light sensitivity.

Do Things Look Different When You Are High?

A 2021 study conducted at the University of Granada used a battery of tests to assess changes in vision associated with marijuana use. Current cannabis users who smoked marijuana recreationally between one and four times per week were recruited to engage in a ten-minute smoke session with their usual cannabis product. The research team put each subject through a battery of visual tests and compared the results to pre-smoking scores. Vision changes noted from smoking marijuana include:

• Increased accommodative lag: In simple terms, this is the brain’s ability to adjust to and process moving objects and changes in distance.
• Greater glare sensitivity and light halos: After smoking marijuana two-thirds of the test subjects were less able to discern objects close to a light source due to an increase in perceived glare and visual halo effects.
• Impaired night vision: 74% of study participants felt that smoking cannabis affected their night vision and diminished their ability to drive at night.

Why Does My Vision Lag When I Get High?

A French study published in Journal of the American Medical Association: Ophthalmology found that the retinal ganglion cells of marijuana smokers had delayed response times as compared to non-smokers. These cells send electrical impulses from the eye to the brain, and a lag in this transmission may account for some of the visual aberrations associated with marijuana.

Can Marijuana Cause Color Changes?

An interesting topic that has so far received minimal attention from the research community is the effects of marijuana on color discrimination. This phenomenon was noted in the early days of cannabis research with a single study published in a 1976 edition of the Perception & Psychophysics. Study participants took the Farnsworth Munsell Hue Color Test before and after drinking alcohol or smoking marijuana. This test is used to determine if a person has a color vision defect and, if so, to identify the specific spectrums that are problematic.

Test results showed that both alcohol and marijuana consumption impaired the user’s ability to tell the difference between blue and various blue-green and blue-purple samples. Alcohol consumption also created some confusion in the yellow-green range, while marijuana made it more difficult to tell red from yellow-red samples.

It should be noted: This research is nearly five decades old and, to the best of the author’s knowledge, there have been no follow-up studies that confirm or negate these findings.

Does Marijuana Help or Hurt Night Vision?

The influence of marijuana’s effect on night vision remains a topic of debate. While the participants mentioned in the University of Grenada study felt their visual perception was compromised at night, several studies report cases of enhanced night vision when under the influence of marijuana.

The first known mention of this phenomenon came from a letter written by M.E. West of the University of the West Indies, which was published in a 1991 edition of Nature. In the letter, West describes a nocturnal boat trip with local Jamaican fishermen who routinely imbibed a cannabis-infused rum that supposedly improved night vision.

After watching the crew thread their way through a maze of coral reefs in near-total darkness, West wrote:

It was impossible to believe that anyone could navigate a boat without a compass and without light in such treacherous surroundings. I was then convinced that the men who had taken the rum extract of cannabis had far better night vision than I had.

In a subsequent interview with the Los Angeles Times, he remarked:

Jamaican fishermen, it seems, have an uncanny ability to see in the dark, and cannabis is the magic ingredient.

A decade later, West’s assertions were put to the test by one of the pioneers of cannabis research, Dr. Ethan Russo. The subjects of Russo’s study were kif smokers living in Morocco’s Rif mountains. (Kif is a blend of tobacco and marijuana, traditionally blended in a ratio of one-to-two.)

Researchers measured two key aspects of each subject’s low-light adaptability and night-vision acuity before and after smoking kif. In a separate evaluation, the subjects were given a 20mg dose of synthetic THC. In both tests, the subjects showed improvements in night vision measures after ingesting THC.

Neither West nor Russo attempted to explain the biological mechanisms behind cannabis-enhanced night vision. No, evidence for the cause of cannabis-enhanced low light perception came from tadpoles!

In 2022, Neurobiology of Disease reported that researchers from McGill University and the Montreal Neurological Institute were initially surprised at the results of an experiment involving tadpoles. When the tadpoles were given a dose of synthetic cannabinoids, they showed a measurable improvement in low-light vision. This ran contrary to the researchers’ predictions, as cannabis is often associated with reduced neurotransmission – an effect that would have dampened the optic nerve signaling and worsened night vision.

The explanation for this unexpected phenomenon was traced to the ability of certain cannabinoids to use CB1 receptor activation to boost the activity of retinal ganglion cells (RGCs). These cells transmit light detection information from the eye to the brain, and stimulation makes them more sensitive to light. The RGC-boosting effects of cannabis allowed the tadpoles to detect faint objects in low light and decreased the eye’s response time to stimulus.

How Does Marijuana Affect Vision When Driving?

There’s strong evidence that marijuana can alter vision and visual performance. This raises the question of what effect these alterations might have on driving performance and safety. Before getting into specifics, we should note that CannaMD, and many other sources, recommend waiting a minimum of 8 hours after marijuana use before getting behind the wheel.

A study published by Psychopharmacology reports:

Cannabis users showed decreased spatial contrast sensitivity under low luminance conditions and increased motion coherence thresholds under all luminance levels as compared to non-users. No difference in temporal contrast sensitivity was found between the groups.

In simple terms:

• “Spatial contrast sensitivity” refers to the eye’s ability to detect and recognize objects separate from the background, like recognizing the profile of a tree in the fog.
• “Motion coherence thresholds” describe the brain’s ability to detect motion, like seeing a car coming in the opposite direction.
• “Temporal contrast” describes the amount of time it takes the optic pathway to relay information on the speed and distance of that oncoming car.

The authors also noted that these changes and delays became more pronounced when greater quantities of marijuana were consumed. A fact that is not surprising to cannabis users.

In a 2020 study titled, “Effects of Smoking Cannabis on Visual Function and Driving Performance”, participants were scored on a driving simulator before and after smoking marijuana. Test results indicated:

• Cannabis use reduced visual acuity (or how large an object must be to be seen), contrast sensitivity (the level of contrast a pattern must have to be seen), and stereoacuity (the ability to judge the distance to an object).
• After smoking cannabis, drivers had a harder time staying in their lane — a result that could be expected when visual acuity and contrast sensitivity are compromised. Drivers also tended to decrease speed and increase following distances behind other vehicles, possibly as compensation for reduced depth perception.

In concluding remarks, the authors claimed that “this study provides the first evidence to show that the visual effects of cannabis could impact driving performance, compromising driving safety.”

Interestingly, this reporting also indicates that the legalization of marijuana does not appear to be associated with an increase in motor vehicle accidents.

As mentioned previously, CannaMD does not advocate driving while under the influence of marijuana and has written on the subject of marijuana and driving safety.