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What Causes Eye Redness in Some Cannabis Users?

March 31, 2021 in General Education

One of the most notable signs of cannabis usage is the presence of red, bloodshot eyes. But, not everyone who uses cannabis experiences this physical response.  Just like everything related to cannabis, there are always differing opinions and explanations for any given topic. In this article I will present the different theories that explain eye redness in some cannabis users. On the bright side, it does not cause any serious health risks. I will leave you to draw your own conclusion about which ones you believe.

Consumption of THC Affects Heart Rate and Blood Pressure

The typical response to the consumption of any cannabis product including edibles, topicals and tinctures, is an increase in heart rate and blood pressure. This is due to the interaction of THC with the cannabinoid receptors located throughout your body.  These increases are comparable to your physiological response to normal physical activities such as exercise or sex.

Your blood pressure level does have an impact on the occurrence of red eyes. People with higher blood pressure need to use a higher THC strain to lower their blood pressure enough to cause intense eye redness. However, those with naturally lower blood pressure may also experience red eyes.

It typically takes about 5-10 minutes for your heart rate to return to normal and for your blood pressure to start to decrease. This causes the blood vessels and capillaries, including the ocular capillaries, to dilate. When the ocular capillaries dilate, blood flow to the eyes increases which makes the white area, known as sclera, look red. Your eyes, in particular, contain very large numbers of cannabinoid receptors. In general, the higher the THC content, the redder your eyes become.

The good news is that the intraocular pressure in your eyes also decreases temporarily. That is why medical cannabis is recommended for glaucoma patients. The main symptom of glaucoma is high intraocular pressure which may result in optic nerve damage and vision loss leading to blindness. In a study, Marijuana Smoking vs Cannabinoids for Glaucoma Therapy, published in the November 1998 Journal of The American Medical Association (JAMA), smoking cannabis was shown to reduce intraocular eye pressure by up to 30%.

However, there are studies which refute these findings. Such a study, CBD in Marijuana May Worsen Glaucoma, Raise Eye Pressure, conducted at Indiana University, was published in Science Daily in December 2018. It found that CBD usage may actually worsen glaucoma by increasing intraocular pressure. Clearly, further research into cannabis usage as a glaucoma treatment is needed before a final recommendation can be made.

Not All People Develop Red Eyes

The THC content of a strain is not the only factor which plays a role in making your eyes red. Each cannabis consumer presents with a unique response to any given cannabis strain. This is based on individual factors which include genetics, gender, frequency of use, your overall health and more.

Cannabinoid Allergy

Another theory is that a small number of cannabis consumers have a minor allergic reaction to the carcinogens in cannabis which can irritate the eyes. Smoking any substance produces carcinogens which can be an irritant and smoking cannabis is no exception.


The final theory is that the pesticides and chemicals that some growers use cause red, bloodshot eyes. For the most part, that is most likely found in the black market where there are no cultivation regulations and no lab testing. While many medical cannabis states are required to adhere to strict guidelines restricting the use of certain pesticides, not all do so.

Some of the chemicals used in the cultivation process leave a chemical, metallic or general unpleasant flavor. Chemical usage may also cause consumers to develop headaches.

Possible Remedies for Red Eyes
  1. Choose strains with low or no THC such as those high in CBD or CBN.
  2. Stay hydrated  by drinking lots of water was suggested by many, although this remedy was refuted as being entirely false by others.
  3. Plan the timing of your cannabis usage to allow plenty of time for your body and your eyes to recover before starting work or attending an important meeting.
  4. Use eye drops especially designed to reduce redness. Those which contain tetryzoline, such as Visine, a vasoconstrictor, work best to reduce blood flow to your eyes.
  5. Wearing sunglasses when outside in direct sunlight may minimize irritation of the dilated blood vessels in your eyes.
  6. Consume foods or beverages that trigger vasoconstriction such as caffeine, licorice or dark chocolate. Splashing cold water on your face, taking a cold shower or placing an ice pack on your eyes also cause vasoconstriction.

How Long Do Your Eyes Stay Red?

The amount of time that the redness persists may take anywhere from 2-12 hours depending on the following factors:

  1. Delivery Method
  • Smoking or vaping; effects kick in within 2-5 minutes, peak within 15-30 minutes and typically wear off within 1-3 hours, depending on the dosage.
  • Edible; effects take 30-90 minutes to kick in, peak after 2-3 hours and can last from 4-12 hours, depending on the dosage.
  1. Tolerance Level
  2. Body Weight
  3. Metabolism
  4. Frequency of Usage; regular users may develop a resistance to red eye

weedmaps, Why Does Weed Make Your Eyes Red?, Why Do Smoking Cannabis and Eating Edibles Cause Red, Bloodshot Eyes? Rae Lland, Oct. 13, 2016, Why Does Marijuana Cause Red Eyes and What To Do About It?, This Is The Real Reason Why Marijuana Makes Your Eyes Red, Sarah Parfitt, Jan. 31, 2017, CBD in Marijuana May Worsen Glaucoma, Raise Eye Pressure, Dec. 17, 2018


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