Red, bloodshot eyes are a common side effect of cannabis consumption. This distinctive change is not caused by smoke irritation, as often assumed, but is a direct physiological response to Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis. When THC enters the bloodstream, this chemical interaction triggers a chain of events within the cardiovascular system that ultimately results in the reddening of the eyes.
The Role of THC and Vasodilation
The cause of cannabis-induced eye redness begins with how THC interacts with the circulatory system. When THC enters the body, it binds to cannabinoid receptors within the endocannabinoid system. This binding initiates a systemic change, causing a temporary decrease in overall blood pressure. To compensate for this drop in pressure, the body initiates a process known as vasodilation.
Vasodilation is the widening of blood vessels throughout the body, which allows blood to flow more freely and helps restore normal blood pressure. This reaction occurs across the entire vascular network, not just in a single area. The relaxation of the smooth muscle tissue in the artery walls increases their diameter. This systemic response is the mechanism behind the visible changes in the eyes.
Ocular Effects and Intraocular Pressure
The visible redness is the localized effect of systemic vasodilation occurring in the eye. The tiny blood vessels, known as capillaries, located in the sclera (the white layer of the eye) expand in diameter. Since the sclera is covered by a thin, transparent membrane, this expansion makes the increased blood flow highly visible, resulting in the characteristic red or bloodshot appearance. The degree of redness often correlates directly with the dose or potency of the THC consumed.
This physiological process also lowers intraocular pressure (IOP), the fluid pressure inside the eye. High IOP is a concern for conditions like glaucoma, which can damage the optic nerve. THC’s ability to dilate blood vessels reduces this pressure, a temporary effect that typically lasts for three to four hours. While this effect has been studied, the short duration and systemic side effects mean cannabis is not a standard treatment for glaucoma.
Addressing the Redness
The red-eye phenomenon is a benign and temporary side effect of THC consumption. The redness typically subsides within one to four hours as the body metabolizes the THC and blood pressure returns to its normal state. The duration can vary based on the method of consumption, the concentration of THC, and the individual’s metabolism.
For those who wish to mitigate the visible redness quickly, over-the-counter eye drops are a common option. These drops often contain a vasoconstrictor compound, such as tetrahydrozoline hydrochloride. This compound works by reversing the vasodilation, causing the tiny blood vessels in the eye to constrict. Simple methods like applying a cold compress to the eyelids or ensuring proper hydration can also help reduce the appearance of redness by encouraging slight vessel constriction.