The physical changes in the eyes following cannabis use, often described as “low eyes,” result from how the psychoactive compound, Tetrahydrocannabinol (THC), interacts with the body’s vascular and muscular systems. This distinctive appearance is caused by two physiological effects: visible eye redness and a subtle drooping of the eyelids. Understanding these effects requires examining THC’s systemic impact on blood flow and muscle tone.
How THC Causes Eye Redness
The visible redness in the white part of the eye, the sclera, is a direct consequence of vasodilation. THC is a potent systemic vasodilator, causing blood vessels to relax and widen throughout the body. This effect occurs because THC interacts with the endocannabinoid system, binding to CB1 and CB2 receptors in the cardiovascular system.
When THC enters the bloodstream, it causes the tiny capillaries on the surface of the eye to expand. This widening increases blood flow, making the vessels, which are normally nearly invisible, much more prominent against the white sclera. This is why the eyes appear “bloodshot” or red.
The redness is a systemic physiological response, not caused by smoke irritation, and occurs regardless of the consumption method (e.g., edibles or vaporizers). This vasodilation typically causes a temporary drop in overall blood pressure. The body compensates by increasing the heart rate, and the vessel dilation is most noticeable in areas with superficial vessels, such as the eyes.
The Mechanism Behind Eyelid Drooping
The “low” or heavy-lidded appearance is medically referred to as ptosis, or drooping of the upper eyelid. This effect is distinct from the redness and is attributed to THC’s muscle-relaxing properties. THC’s interaction with the central nervous system causes the smooth muscles responsible for maintaining eyelid elevation to relax.
This relaxation affects the levator palpebrae superioris muscle and Müller’s muscle, which lift the upper eyelid. When these muscles lose tone, the eyelid margin lowers slightly. Mild ptosis is a frequently observed side effect of cannabis consumption.
One study involving impaired drivers found that a high percentage of individuals with THC exhibited signs of ptosis during field sobriety tests. While the precise neurological pathway linking cannabinoids to this specific muscle relaxation is still being explored, the resulting mild ptosis contributes significantly to the perceived “low” look.
The Science of Lowered Eye Pressure
The vasodilation caused by THC results in a reduction in intraocular pressure (IOP), the fluid pressure inside the eye. THC can lower IOP by an average of 20 to 30 percent in most users. This effect is why the compound has been studied for treating glaucoma, a condition characterized by high IOP that damages the optic nerve.
This pressure drop is achieved through two main mechanisms related to the eye’s fluid dynamics. THC interacts with CB1 receptors in the ciliary body and the trabecular meshwork, structures that regulate the clear fluid inside the eye called aqueous humor. One mechanism involves the widening of blood vessels, which increases the outflow, or drainage, of the aqueous humor.
The other factor is a potential reduction in the production of aqueous humor by the ciliary body. This dual action reduces the fluid volume and the resistance to its exit, lowering the internal pressure. However, the pressure-lowering effect is temporary, typically lasting only three to four hours, so it is not currently a frontline treatment for chronic conditions like glaucoma.