The interaction between cannabis and the visual system is often simplified to “weed eye,” which describes the most observable physical change. This refers to the acute ocular effects following the use of cannabis products containing \(\Delta^9\)-tetrahydrocannabinol (THC). THC is the primary psychoactive compound responsible for mind-altering effects, but it also acts on receptors found throughout the eye and surrounding tissues. These effects are a biological response, ranging from temporary alterations in visual perception to measurable shifts in internal ocular pressure.
Immediate Visual and Perceptual Symptoms
One of the most noticeable physical reactions to THC consumption is ocular redness, presenting as a bloodshot appearance in the whites of the eyes. Medically, this is known as conjunctival injection, caused by the dilation of blood vessels near the eye’s surface. This symptom is a highly recognizable sign of recent cannabis use.
The visual experience can also be temporarily altered. Some users report heightened sensitivity to light (photophobia), while others perceive colors as more vivid or intense. Additionally, THC can affect the brain’s ability to process visual information, leading to slight blurring, reduced contrast sensitivity, and minor impairment of depth perception.
These perceptual changes are often accompanied by dryness and irritation on the surface of the eye. This discomfort results from a reduction in the eye’s natural tear production. Studies also indicate that the speed at which the retina sends signals to the brain can be delayed, which may contribute to a temporary reduction in visual acuity and slower reaction time to moving objects.
The Physiological Mechanism of Ocular Change
The effects of THC on the eye are mediated by the body’s Endocannabinoid System (ECS), a complex network that regulates various bodily functions. Cannabinoid receptors, specifically the CB1 subtype, are present in numerous ocular tissues, including the ciliary body, the trabecular meshwork, and the retina. THC, an exogenous cannabinoid, activates these receptors upon consumption.
Ocular redness occurs because THC acts as a peripheral vasodilator, triggering a widening of blood vessels throughout the body, including those in the conjunctiva. When these vessels expand, more blood flows through them, making them prominent and giving the eye its characteristic bloodshot appearance. This vasodilation is a localized response to THC binding to receptors in the eye’s vascular tissue.
The sensation of dry eyes is also related to CB1 receptor activation. CB1 receptors are located in the nerve fibers that innervate the lacrimal gland, which produces the aqueous layer of the tear film. When THC activates these receptors, it reduces the gland’s secretion, resulting in decreased tear volume and the feeling of dryness and irritation.
Animal models suggest the effect on tear production may exhibit a sex-dependent difference. In male mice, THC activation of CB1 receptors reduced tearing, while the effect was less pronounced in female mice. This variation is theorized to relate to a higher concentration of CB1 receptors found in the lacrimal glands of male subjects.
Cannabis and Intraocular Pressure Management
Beyond the acute, visible effects, one of the most studied physiological changes is the temporary reduction in Intraocular Pressure (IOP). IOP is the fluid pressure inside the eye, and its elevation is a major risk factor for glaucoma, a disease that can lead to permanent damage of the optic nerve. Studies consistently show that THC can lower IOP by as much as 20% to 30% in both healthy individuals and those with glaucoma.
The pressure-lowering effect is achieved through THC’s action on CB1 receptors located in the ciliary body and the trabecular meshwork. The ciliary body produces aqueous humor, the fluid that maintains IOP, and THC activation reduces the rate of this fluid’s formation. THC may also enhance the outflow of the aqueous humor through the trabecular meshwork, further contributing to the drop in pressure.
Despite the measurable IOP reduction, cannabis is not recommended by major ophthalmology organizations as a treatment for glaucoma. The primary limitation is the short duration of the effect, which typically lasts only three to four hours. Effective glaucoma management requires continuous IOP control 24 hours a day, necessitating dosing six to eight times daily.
Frequent consumption would lead to constant psychoactive impairment, making it impractical for daily life, including driving or operating machinery. Furthermore, THC is a systemic vasodilator that lowers overall blood pressure, which could reduce blood flow to the optic nerve. This reduction in blood supply might counteract the benefit of lower IOP, as reduced blood flow to the optic nerve is a factor in glaucoma-related damage.