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. 

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:

Why does marijuana make your eyes red?

The vasodilation behind your red eyes is triggered by the main psychoactive ingredient tetrahydrocannabinol (THC). THC is just one of 113+ active compounds in marijuana, known as cannabinoids. These cannabinoids interact with cannabinoid receptors throughout your body, and in particular, your eyes.

When THC binds to cannabinoid receptors, it lowers our blood pressure, which in turn causes the dilation or expansion of blood vessels and capillaries. In the case of your eyes, the ocular capillaries dilate, causing an influx of blood flow to the area, creating a red appearance of the sclera (aka the white of the eyes). That’s why we call it “bloodshot” eyes.

If you’ve ever wondered “do edibles make your eyes red too,” this explanation should provide some clarity. As edibles, just like topicals and tinctures, can also contain THC, they too have the potential to cause red eyes through vasodilation.

For glaucoma patients, the THC induced vasodilation of ocular capillaries also temporarily reduces intraocular pressure. High intraocular pressure is the main symptom of glaucoma and can contribute to optic nerve damage and vision loss. Therefore, reducing this pressure is a key priority of glaucoma treatment. One study, published in the Journal of the American Medical Association, reported that smoking marijuana reduced intraocular eye pressure by up to 30%.

Red Eyes One Day, but Not the Next

While red eyes are a dead giveaway that someone has been smoking marijuana, it’s not always a guaranteed symptom. That’s because each strain of marijuana has a different concentration of THC. Hence, someone may experience intensely red eyes after ingesting a high THC strain, but barely notice the difference after a low THC strain.

However, there’s more at play than just THC content. As you may have experienced yourself, the same cannabis strain can have different effects on different people. These varying effects are due to a number of individual factors, including gender, genetics, and overall health.

Whether a person experiences red eyes or not is primarily influenced by their blood pressure. For instance, people with high blood pressure need more THC to lower their blood pressure enough to cause intense bloodshot eyes. Although, those with naturally low blood pressure can quite easily experience terminator red eyes.

With that being said, people with allergies to marijuana or smoke, in general, can experience exacerbated symptoms. But for users with an allergy, typically, red eyes are the least of their concerns.

Treat With Eye Drops

Over-the-counter eye drops designed to treat allergies, redness, and itching are the most effective way to alleviate red eyes. Basically, all variations of eye drops contain tetryzoline, which is an alpha-agonist that constricts blood vessels. Essentially, this reverses the dilation caused by THC, which then reduces blood flow to the eyes and the accompanying redness.

Home Remedies

Other items you might have lying around the house can also trigger vasoconstriction. For instance, stimulants such as caffeine can help induce vasoconstriction. Therefore, drinking a fresh cup of coffee or enjoying a few squares of dark chocolate might help alleviate red eyes.

Cold water is also a strong vasoconstrictor. It forces the body to send blood toward our core to protect vital organs as part of a survival instinct. Splashing cold water on your face or placing an ice pack across your eyes can achieve this effect. If you’re brave enough, a cold shower or ice bath will also do the trick.