Red or bloodshot eyes are a common and noticeable side effect of cannabis use. This distinct change in eye appearance is often one of the first indicators of recent consumption.
THC’s Effect on Eye Redness
Tetrahydrocannabinol (THC), the primary psychoactive compound found in cannabis, is responsible for the reddening of the eyes. THC interacts with the body’s endocannabinoid system, specifically binding to cannabinoid 1 (CB1) receptors located throughout the body, including in the eyes and blood vessels. This interaction leads to a temporary decrease in blood pressure.
The drop in blood pressure causes vasodilation, which is the widening of blood vessels. In the eyes, this vasodilation particularly affects the tiny blood vessels and capillaries on the surface of the eye, known as the conjunctiva. As these blood vessels expand, more blood flows into them, making them more prominent and giving the eyes their characteristic red or bloodshot appearance. This physiological response occurs regardless of the method of consumption, meaning edibles or other forms of cannabis can also cause red eyes, not just smoking.
Other Causes of Redness
While THC’s physiological effects are the main cause of red eyes, other factors can contribute to or worsen the redness, especially when cannabis is smoked. Smoke from any source, including cannabis, can act as an irritant to the delicate tissues of the eyes. This irritation can lead to surface inflammation and further visible redness.
Cannabis use can also lead to temporary dry eyes, a condition where the eyes do not produce enough tears or the tears are of poor quality. This reduction in tear production is believed to involve THC’s effect on the lacrimal glands, which produce tears. Dryness causes discomfort, itching, and can exacerbate the appearance of redness.
How Long Red Eyes Last and What Helps
The redness in the eyes caused by cannabis is generally temporary, typically lasting between one to four hours. The duration can vary based on factors such as the THC concentration in the cannabis consumed, the amount used, and an individual’s metabolism and tolerance. Higher THC content often correlates with more pronounced and longer-lasting redness.
Several practical methods can help alleviate red eyes. Over-the-counter eye drops containing ingredients like tetrahydrozoline or brimonidine, work by constricting the blood vessels in the eyes, thereby reducing the visible redness. Staying well-hydrated by drinking plenty of water can also help by reducing overall dryness. Applying a cold compress to closed eyelids for about 10 minutes can also help constrict blood vessels and soothe irritation.
Understanding the Redness: Is It Harmful?
Eye redness from cannabis is generally not harmful and is a temporary cosmetic side effect that resolves as the effects wear off. The vasodilation property of THC, which causes the redness, also leads to a decrease in intraocular pressure (IOP), the pressure inside the eye.
This reduction in IOP is a medically relevant effect, as it is a key factor in treating conditions like glaucoma, a disease characterized by increased eye pressure. While cannabis has been studied for its potential to lower IOP, it is not a recommended primary treatment for glaucoma due to its short duration of action and psychoactive side effects, which would require frequent consumption.