Article written by
Dipak HemrajHead of Research and Education
Content reviewed by
Dr. Lewis JasseyMedical Director - Pediatric Medicine
Table of contents
Red eyes from cannabis use tend to last one to two hours; this may be up to three or four hours for some people.
Red eyes are often the most telling sign that someone has ingested cannabis. Some people experience it every time they use marijuana, and others may only experience it occasionally. This can be due to differences in body chemistry and the differences between cannabis strains. However, red eyes from cannabis use are rarely ever anything serious to worry about.
Causes of Red Eyes From Cannabis Use
One of the main active components in cannabis is tetrahydrocannabinol (THC). When cannabis is ingested, this compound enters the bloodstream and interacts with the body’s endocannabinoid system (ECS). This system contains receptors spread throughout the body, which cannabinoids like THC bind to, and these receptors are referred to as CB1 and CB2.
The red-eye effect is caused by THC molecules binding to CB1 receptors in the brain and eyes. This induces vasodilation in the eye’s blood vessels, causing an increase in blood flow and a decrease in blood pressure. The increase in blood flow caused by the dilation of blood vessels makes the eyes appear red.
Smoking cannabis has been reported to decrease intraocular pressure (pressure in the eye) by up to 30%. This decrease in intraocular pressure is why cannabis can be used to help with eye conditions such as glaucoma. Glaucoma is a class of eye diseases that impacts the optic nerve and can lead to blindness; it is often caused by high intraocular pressure (IOP).
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Does Ingestion Method Matter?
There are many different methods of cannabis ingestion. Common methods include smoking, vaping, and oral administration, such as edibles. Red eyes from cannabis use are directly linked to the THC concentration in cannabis. This means that regardless of the method of ingestion, THC molecules from cannabis will enter the bloodstream and bind to CB1 receptors in the eye. Because the appearance of red eyes after ingesting cannabis is directly linked to THC, cannabis strains high in THC are more likely to cause red eyes than those low in THC.
The main difference between ingestion methods is the time it takes to appear. Ingestion by smoking or vaping tends to result in red eyes quickly as the THC moves directly into the bloodstream, rapidly entering the blood vessels in the brain and eyes. Irritation caused by smoke may also cause eyes to red, but this is not a major factor. Ingestion via edibles may result in a slower onset of red eyes as the THC must pass through the liver before entering the bloodstream.
When Will the Eye Redness Go Away?
Typically, red eyes caused by cannabis ingestion will go away one to two hours after onset. However, this will differ based on how quickly your body metabolizes the THC and the concentration of THC in the particular cannabis strain; the more THC in the strain, the longer red eyes can last. Red eyes due to marijuana can last three to four hours for some people.
How to Get Rid of Red Eyes From Marijuana
Over-the-Counter Eye Drops
Over-the-counter (OTC) eye drops designed for allergy treatment are the most common method to reduce red eyes caused by marijuana. Eye drops used to treat allergic reactions contain a molecule called tetryzoline (also known as tetrahydrozoline). Tetryzoline is a vasoconstrictor; it constricts the dilated blood vessels in the eye. Constricting the blood vessels causes a reduction in blood flow, lessening the appearance of red eyes.
Caffeine may reduce eye redness by a similar mechanism to eye drops. Caffeine causes vasoconstriction, reducing blood flow in the eyes and decreasing redness. You may find that consuming one of the following beverages will help reduce eye redness.
- Energy drinks
Coldness causes the blood vessels in the peripheral body parts (like the eyes) to constrict, reducing blood flow in these areas. This is a survival tactic the body naturally undergoes when cold to send most of the blood to the vital organs such as the brain and the heart. This is why people often experience pale or blue fingers and toes when cold. Try placing a cold compress over your eyes or splashing some cold water on your face to reduce red eyes. Alternatively, if you are brave, you could take a cold shower or jump in an ice bath.
If you cannot reduce the redness in your eyes, you could wear sunglasses. This will not make the redness disappear, but the sunglasses will hide it. Unfortunately, this is not a good solution for the winter.
Use a Low-THC Cannabis Strain
Using a low-THC strain of cannabis will not reduce already red eyes; however, it may prevent the onset of red eyes. As vasodilation in the eyes is directly linked to THC concentration in cannabis, ingesting a cannabis strain that is low in THC will reduce the chances of red eyes occurring.
Frequently Asked Questions
How Do I Clear the Redness in My Eyes After Smoking Weed?
The best way to quickly clear the redness from your eyes after smoking cannabis is to try over-the-counter eye drops designed for allergy and red eye treatment. Alternatively, consuming caffeine or placing a cold compress over your eyes may help to reduce the redness. Each of these solutions works by constricting the blood vessels in the eye to reduce blood flow, decreasing redness.
Does Sleeping Help Reduce Red Eyes From Weed?
Sleep does not reduce red eyes from marijuana. However, when you are asleep, your body will metabolize the THC; this is the compound that causes the redness. Once THC is metabolized, it will no longer be bound to the CB1 receptors in the eyes, therefore, no longer causing redness. For red eyes to completely subside during sleep, you need to sleep for a few hours.
Do Edibles Cause Red Eyes Like Smoking Marijuana Does?
Yes, edibles can cause bloodshot eyes like smoking marijuana does. THC from cannabis in the circulatory system causes red eyes, not by the smoke as some think (although some irritation may occur due to the smoke). Red eyes from edibles may take longer to appear as THC has to go through the liver before entering the bloodstream, whereas THC ingested via smoking enters the bloodstream quickly through the lungs.
Red eyes caused by marijuana is a result of the THC concentration. If the marijuana used in the edibles has a low concentration of THC, the risk of developing red eyes is reduced. This is the same regardless of how it is ingested.