The common occurrence of red eyes following cannabis use is a highly recognizable symptom. This visible redness, known medically as conjunctival injection, is a direct physiological response to the compounds within the plant. The phenomenon occurs regardless of the consumption method, whether smoking, vaporizing, or eating edibles. This reaction is systemic, meaning it happens throughout the body, and is a temporary side effect of chemical interaction.
THC’s Impact on Blood Vessels
The characteristic redness is primarily caused by the psychoactive compound delta-9-tetrahydrocannabinol (THC), which enters the bloodstream upon consumption. THC interacts with the body’s endocannabinoid system, triggering vasodilation—the widening or relaxation of blood vessels throughout the body.
This dilation action causes a temporary drop in overall systemic blood pressure. The cardiovascular system compensates for this drop by allowing blood to flow more freely. The effect is most visible in the eyes because the capillaries in the conjunctiva—the clear membrane covering the white part of the eye—are tiny and close to the surface. When these microscopic vessels widen, the increased blood volume makes the normally faint red lines much more prominent and noticeable, creating the bloodshot appearance. The degree of redness is directly related to the potency and amount of THC consumed.
The Effect on Internal Eye Pressure
In addition to causing superficial redness through vasodilation, THC also affects the internal mechanics of the eye. The compound binds to cannabinoid receptors, specifically CB1 receptors, which are present in the eye’s structures. This interaction leads to a reduction in intraocular pressure (IOP), the fluid pressure inside the eyeball.
This decrease in pressure is a distinct physiological event from the superficial vasodilation that causes redness. The pressure drop occurs because THC reduces the production of aqueous humor, the fluid that fills the front part of the eye, and may also increase its drainage. This effect on IOP is why cannabis was historically studied for potential application in managing glaucoma, a disease characterized by dangerously high intraocular pressure.
How Long the Redness Lasts
The duration of the red eye effect is tied to how long THC remains active in the body. For most people, the redness is temporary and subsides relatively quickly, typically lasting between one to two hours after consumption. For users who consume higher concentrations of THC or use edibles, the redness may persist for up to three or four hours.
If a person wishes to reduce the visibility of the redness sooner, simple measures can be taken. Over-the-counter eye drops that contain a vasoconstrictor, such as tetrahydrozoline, work by actively shrinking the dilated blood vessels in the eye. Staying hydrated can also help, as dehydration can sometimes exacerbate eye dryness and irritation.