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Why Does Weed Make Your Eyes Red: The Different Reasons

Cannabis 101 · By Aaron Girard · Updated · 5 min read

red eyes from smoking weed
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Altered coordination, delayed response time, increased heart rate, and dry mouth are some tell-tale signs a person has just used weed. In this post, our focus is on red or bloodshot eyes.

You’re probably wondering why weed makes your eyes red. Some might assume that it’s because of the smoke entering the eyes.

However…

The main cause is how THC acts on blood vessels in the eye. Below, we cover the science and whether it’s dangerous.

This article is for adults 21+ where cannabis is legal. Under US federal law, marijuana remains a Schedule I drug outside the narrow categories a 2026 DEA final rule moved to Schedule III, and state laws differ.

The Science Behind Red Eyes

Red eyes come from the way cannabinoids interact with the body’s endocannabinoid system.

Tetrahydrocannabinol (THC), the primary psychoactive compound in marijuana, wields its influence by binding to cannabinoid receptors, triggering a cascade of effects.

One such effect is a process known as vasodilation, or widened blood vessels. A review of cannabis and the eye explains that THC causes vasodilation by binding cannabinoid receptors in the eye.

As the small vessels on the white of the eye widen, more blood flows through them and the eyes look red. Doctors call this conjunctival injection, and the review describes it as acute and temporary.

Histamine Release and Inflammation

You may read that THC triggers histamine release that reddens the eyes. We could not find credible evidence for this, and the eye-focused review above does not mention it.

The well-documented cause is vasodilation, so treat histamine claims with caution.

The Connection with Blood Pressure

Cannabis affects blood pressure in more than one way. NIDA notes that cannabis can increase heart rate and blood pressure right after use.

The American Academy of Ophthalmology, on the other hand, says marijuana lowers blood pressure throughout the body. The widening of blood vessels is the part that matters most for red eyes.

If you have heart problems, these blood pressure and heart rate changes are worth discussing with your doctor.

Concentrates carry much more THC than flower. You may be wondering what is a shatter and the possible dangers of using one, so you might as well read our article about that.

Dry Eyes and Irritation

Cannabis may also reduce tear production. In the review above, studies in mice found THC reduced tearing, and some medical cannabis users report dry eyes.

When the eyes lack sufficient moisture, the conjunctiva, the thin, transparent layer covering the white part of the eye, can become more exposed and prone to irritation.

Dry, irritated eyes plus widened blood vessels add up to a red-eye look. Smoke can irritate the eyes too.

Impact on Intraocular Pressure

Intraocular pressure refers to the pressure within the eyeball, which is regulated by a balance of fluid production and drainage.

THC can lower intraocular pressure, but only for about three or four hours, according to the American Academy of Ophthalmology.

High eye pressure is a key risk factor in glaucoma, but the AAO does not recommend marijuana to treat it. It warns that marijuana could reduce blood flow to the optic nerve, which could cancel out the benefit.

Is It Dangerous to Have Red Eyes from Smoking Weed?

Photo from Deposit Photos

Red eyes after using marijuana are common. But are bloodshot eyes harmful to health?

Generally, they’re not dangerous. The redness happens because the blood vessels in your eyes widen after you consume THC.

It’s usually a short-lived effect and not a big concern for most people.

Still, be aware of your own health. If you have heart problems, the heart rate and blood pressure changes from cannabis might be risky. People who tend to have dry or irritated eyes might also find the redness uncomfortable.

What To Do When You Have Red Eyes from Smoking Marijuana

If you find yourself with red eyes after smoking weed, there are simple steps you can take:

Stay Hydrated

Drinking water supports your overall health and may help you feel less dry.

It won’t stop THC from widening blood vessels, but it can make dry, irritated eyes more comfortable.

Use Eye Drops

Lubricating eye drops, sometimes called artificial tears, can relieve dryness and irritation.

Over-the-counter drops can soothe your eyes and make them more comfortable.

Redness-relief drops are also sold over the counter. Follow the label directions on how often to use them.

Keep a bottle handy for those moments when your eyes could use some extra moisture.

Avoid Rubbing

When your eyes feel dry or itchy, it’s a natural impulse to want to rub them. However, rubbing your eyes can worsen the situation and lead to increased redness.

The friction caused by rubbing can further irritate your eyes, intensifying the red appearance.

Instead of rubbing, try blinking more often to help spread your natural tears or use lubricating eye drops.

Limit Usage

If you’re concerned about the red-eye effect, you might consider using less cannabis or lower-THC products.

Avoiding smoke can also reduce eye irritation, but edibles with THC can still cause red eyes.

Conclusion

Weed makes your eyes red mainly because THC widens blood vessels in the eye. It isn’t only a reaction to harsh smoke.

Reduced tears and smoke irritation can add to the redness. For most people, it is temporary and harmless.

FAQs

How can you get rid of red eyes from being high?

There is no instant fix, but lubricating or redness-relief eye drops and a cold compress can help. The redness fades as the THC effect wears off.

How long do red eyes last?

Red eyes from cannabis are temporary and usually fade within a few hours. How long depends on the dose and the person.

Can eating edibles make your eyes red?

Yes. Edibles that contain THC can also make your eyes red, because the redness comes from THC, not just smoke.

Sources

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