Why Does Smoking Cannabis Cause Red, Bloodshot Eyes?

When smoking cannabis, I get red, bloodshot eyes. Is this the smoke that causes it? What can I do to stop it from happening?

Marvin P.

Bloodshot eyes have always been a sign that you’ve been smoking cannabis. However, it’s not the smoke that does this. It’s tetrahydrocannabinol (THC) and other active cannabinoids that cause “vasodilation” and may also make you feel dizzy.

THC lowers blood pressure, which in turn causes blood vessels and capillaries to dilate, which causes blood flow to increase, and can make the whites of the eyeballs appear red. THC and other cannabis compounds interact with cannabinoid receptors, which are part of the endocannabinoid system (ECS). They are found throughout your body, including your eyes.

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Cannabinoids bind to cannabinoid receptors and cause the dilation (widening) of the blood vessels, increasing the blood flow to these areas. This in turn decreases the blood pressure, causing the red appearance and dizziness.

If you try other types of cannabis (other than smoking) with substantial amounts of THC, such as edibles, you’ll have the same result even without the smoke.

So to prevent red-eye, one solution may be to use a low-THC strain of marijuana. But once you’ve got a lot of THC in your system, there’s not a lot you can do to stop red-eye from happening. You can try eye drops, use cold compresses or eye drops, or even drink water since sometimes dehydration can cause or contribute to having red eyes. These may help.

And of course, there’s always the old standby of wearing sunglasses. You’ll look cool and nobody will see the color of your eyes.

Also, keep in mind that the reddening of your eyes is not harmful and they will eventually make the transition back to your usual pearly white.


As the blood pressure lowers when you smoke cannabis, the blood vessels and capillaries dilate, including the ocular capillaries. The increased blood flow to your eyeball causes the red appearance or bloodshot eyes. There are no serious health risks associated with your sudden red-eyed circumstance.

We should also point out that in some cases cannabis may actually help our eyes and vision. There is some research that suggests the neuroprotective and antioxidant properties of CBD and THC may be able to help our vision and provide some protection against common diseases of the eye and optic nerve.

One area of eye health where cannabis may have a supporting role to play is glaucoma — a condition of increased pressure within the eyeball, causing gradual loss of sight. Usually, glaucoma is treated with medicated eye drops, which help to bring down intraocular pressure to normal levels. However, some glaucoma patients and eye specialists report that cannabis can also help reduce intraocular pressure and help preserve eyesight.

But on the downside, the effects may only last three or four hours, so you would have to consume marijuana a number of times during the day to get its effects around the clock. While cannabis does lower eye pressure, it also lowers blood pressure. This might result in reduced blood supply to the optic nerve, which in turn might harm the optic nerve.

More research needs to be done. For now, you should start by consulting with an ophthalmologist. At the same time, get their advice on using medical marijuana as a complementary or supplemental therapy to traditional medication.

Annie HMD

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