Why Does Smoking Cannabis Cause Red, Bloodshot Eyes?

Bloodshot eyes have always been a telltale sign that you’ve been smoking weed. But contrary to popular belief, it isn’t the smoke that causes the redness. Using cannabis in other forms can cause the same effect.

You probably remember being a teenager with concerns that your red, bloodshot eyes would give away that you were high. Maybe you’d dash up to your room to bypass your parents, or if you planned ahead, you kept a bottle of eye drops in your coat pocket. As an adult, you likely worry less but being found out, but the science behind this classic dead giveaway is interesting.

Those red eyes are the result of vasodilation, caused by the effects of THC (delta-9 tetrahydrocannabinol) and other cannabinoid compounds on blood vessels in the eye. If you’re concerned about this side effect, here’s why that happens and what you can do about it.

Cannabis Causes Changes in Blood Pressure and Blood Flow

Cannabis can cause a wide range of effects on the body. Many are helpful, such as relieving chronic pain or offering some much-needed relaxation after a stressful day. Others, not so much, including the paranoia and anxiety some people feel when they’ve consumed a lot of cannabis containing THC.

All of these effects are due to the interactions between the many active compounds in cannabis and the endocannabinoid system (ECS), a network of cell receptors found throughout the body. These receptors respond not only to the cannabinoid chemicals our bodies naturally produce, but also to those in the cannabis sativa plant.

The endocannabinoid system works to ensure the healthy functioning of many of the body’s subsystems, including the veins and capillaries that support blood flow throughout the body. When you consume cannabis, the compounds it contains can activate ECS receptors in the blood vessels, including those in the eyes. That can cause a variety of changes in blood pressure and blood flow.

Consuming cannabis can cause a temporary rise in blood pressure. But THC also acts on ECS receptors to lower blood pressure. That in turn causes blood vessels and capillaries to dilate, so that blood flow increases. The increased flow of blood to vessels in the eyeball can make the sclera, or the “white” of the eye, look red. But there are no serious health risks associated with your sudden, red-eyed appearance.

Cannabis Can Benefit Your Eyes

In some cases, cannabis may actually help our eyes and vision. Some research suggests the neuroprotective and antioxidant properties of CBD and THC may be able to support vision and provide some protection against common diseases of the eye and optic nerve.

One area of eye health where cannabis may play a beneficial role is glaucoma—a condition of increased pressure within the eyeball, causing a gradual loss of sight. Glaucoma is usually 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. And since cannabis lowers blood pressure as well as eye pressure, a drop in blood pressure could result in reduced blood supply to the optic nerve. That can cause temporary or even permanent damage to the nerve.

More research is necessary to fully understand the effects of cannabis and its individual compounds on the eyes and optic nerve. But if you’re considering cannabis for glaucoma or other eye conditions, it’s wise to consult with an ophthalmologist about using medical marijuana as a complementary or supplemental therapy to traditional medication.

You Don’t Have to Smoke to Get Red Eyes

Because bloodshot eyes are caused by the psychoactive cannabinoid THC, you’ll experience the same effect whether you smoke or consume other cannabis products with a substantial amount of THC, such as edibles, tinctures or concentrates. Products containing only CBD won’t cause red eyes, since CBD isn’t the cannabinoid that’s responsible for blood pressure changes and vasodilation. It may not happen with full-spectrum products containing very low amounts of THC either. This research has a limit on discussing whether Delta-8 and Delta-10, milder relatives of THC, also cause bloodshot eyes.

What Can You Do About Bloodshot Eyes?

Once you have a considerable amount THC in your system, there’s not a lot you can do to stop red-eye from happening. Vasodilation is simply one of the effects of this cannabis compound, and it gradually resolves as the THC leaves the body.

To reduce the effect, you might try eye drops designed to reduce eye redness, or use cold compresses on your eyes. Even drinking water might help, since dehydration can also cause or contribute to having red eyes. And of course, there’s always the old standby of wearing sunglasses.

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