Why Weed Make Your Eyes Red? Scientifically Explained

why weed make red eyes

Modern scientific investigation into red eyes and weed is finally solved. We now know that cannabinoids trigger reactions within blood cells. THC specifically triggers vasodilation or the opening of blood cells. Wider cells mean more blood flow and more blood underneath the surface of the eyes.


Red and irritated eyes have long been the bane of the weed smoker concerned with discretion. This relationship between red eyes and weed is so well known that bloodshot eyes are what we expect from the stereotypical stoner. 

But while the cannabis community has long known about the red eyes, there have always been some underlying questions. Why does weed make your eyes red in the first place? Are there risks to bloodshot eyes from cannabis consumption? And, perhaps most importantly, are there ways to fix red eyes after you’ve smoked up?



The Science: Why Does Weed Make Your Eyes Red?

According to “Acute Effects of Smoked and Vaporized Cannabis in Healthy Adults Who Infrequently Use Cannabis,published in 2018, the side effects post-session depend on dose size and route of administration. This small study, working with 17 cannabis-friendly participants, tested two THC doze sizes (10 mg and 25 mg) and smoking versus vaping.

The researchers discovered that the larger THC dose (smoked or vaped) led to more red and irritated eyes. No surprise there. What’s more? Vaping THC increased red and irritated eyes no matter the dose. So those participants who vaped the big dose of 25 mg experienced significantly more bloodshot eyes than all other groups.

While it’s all well and good to know that the route of administration and dose size impact bloodshot eyes, what is the biological mechanism at play here? Long ago, stoners theorized that it was just the smoke that irritated the eyes, but thanks to the rise of smoke-free weed (like edibles and vape pens), we know this is no longer a possible explanation.

The real reason why red eyes from weed are common is thanks to the vasoreactivity of cannabinoids. Vasoreactivity describes different reactions in blood vessels to different compounds. Some cannabinoids, like THC, are vasodilators, or compounds that open up blood vessels. Other cannabinoids are vasoconstrictors (making blood vessels smaller).

THC is a known vasodilator, meaning it relaxes and widens blood vessels. This allows for increased blood flow. Technically, THC causes vasodilation throughout your body, from your cardiovascular system to your digestive system, but we generally don’t see the evidence. 

Only when the blood cells in our eyes vasodilate after consuming weed that the telltale red eyes appear. This is because the blood vessels across the surface of our eyes flood with bright red blood.



How To Reduce The Risk Of Bloodshot Eyes 

  1. Route of Administration: According to the 2018 study above), people who vaped cannabis experienced more extreme redness than those who smoked. If you have a choice, you may want to vape.
  2. Smaller Doses: Another point made in the 2018 study was the effect of dose size. Smaller doses led to less eye-irritation.
  3. Strain Selection: Some strains are known for their ability to trigger red eyes and irritation. Dig into strain reviews to find those without red eyes noted in the side effects.

How To Fix Red Eyes After Smoking Weed

Weed causes red, irritated eyes — there is no getting away from it. But there are a few cover-ups! 

  1. Eye Drops: The tried and true method for reducing redness is the application of eye drops. Brands like Visine have arguably stayed in business thanks to the millions of stoners using the product. Visine lubricates the eyes to reduce irritation and may help constrict the blood vessels.
  2. Sunglasses: Worst case scenario? Throw on a pair of sunglasses. If it’s nighttime, of course, this solution isn’t ideal. But, should you be outside on a sunny day, sunglasses cover up any eye irritation until it goes away.
  3. Wait it Out: While perhaps not an option for everyone, the only real way to reduce redness after smoking weed is to wait it out. As the effects from your smoke session wear off, so too will the vasodilation.

Bottom Line

Try experimenting with different strains, cannabinoids commendations, and products to find one with a low risk for eye irritation. While you might not eliminate the risk altogether, by lowering the dose, changing the route of administration, and always carrying a bottle of Visine, you can reduce your chances of getting caught.

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