Cannabis is a widely used substance, with an estimated 219 million users globally in 2021. While its impact on mood and perception is well-documented, its effects on vision and eye health are less commonly discussed. This article explores how cannabis can influence visual perception and eye physiology.
Immediate Visual Effects
Cannabis consumption can lead to several noticeable changes in vision and eye appearance. A common immediate effect is conjunctival hyperemia, often referred to as “red eyes,” which occurs due to increased blood flow to the conjunctiva. This vasodilation is a reliable physical indicator of recent cannabis use.
Cannabis can also alter visual processing, leading to impaired depth perception and reduced peripheral vision. It can decrease contrast sensitivity, especially in low light, and deteriorate stereoacuity, affecting near and far vision. Some individuals may also experience increased light sensitivity, known as photophobia. The effect on pupil size can vary, with some studies reporting constriction and others dilation.
Underlying Mechanisms
The visual effects of cannabis stem from the interaction of cannabinoids, primarily delta-9-tetrahydrocannabinol (THC), with specific receptors in the body. The human body contains an endocannabinoid system with cannabinoid receptors, mainly CB1 and CB2, found throughout the central nervous system, including the brain and eyes. THC exerts its psychoactive effects by binding to CB1 receptors, which are abundant in brain regions involved in movement control, memory, mood, and perception.
Cannabis also influences blood flow to the eyes. The interaction of cannabinoids with receptors can modulate neural pathways involved in visual processing, potentially altering how visual information is transmitted from the retina to the brain.
Cannabis and Eye Health
Cannabis has been explored for its potential therapeutic applications in eye health, particularly for managing glaucoma. Glaucoma is a condition where the optic nerve is damaged, often due to elevated intraocular pressure (IOP). THC has been shown to temporarily reduce IOP, an effect studied for decades. However, this reduction is short-lived, lasting 3 to 4 hours, requiring frequent dosing to maintain a consistent effect.
Despite this temporary IOP reduction, major ophthalmological organizations, including the American Academy of Ophthalmology, do not recommend cannabis as a primary treatment for glaucoma. This is because constant high doses of THC can lead to significant side effects, including impaired mood and mental clarity, making it impractical for daily management. Cannabis can also lower overall blood pressure, which might reduce blood flow to the optic nerve and potentially counteract the benefit of lowered IOP in glaucoma patients. Some research suggests that long-term cannabis use may negatively impact visual acuity and retinal function, though more studies are needed.
Duration of Visual Alterations
The duration of visual alterations from cannabis varies depending on several factors, including the method of consumption, dosage, individual metabolism, and tolerance. When cannabis is smoked or vaped, the immediate visual effects manifest quickly and may last for 1 to 4 hours. For edibles, the onset of effects is slower, taking 30 minutes to 2 hours, as THC is processed through the digestive system and liver.
Once the effects of edibles begin, they last longer than inhaled cannabis, ranging from 4 to 8 hours, with peak effects occurring 2 to 4 hours after consumption. Higher doses or lower individual tolerance can extend these effects, up to 12 hours or even 24 hours. The prolonged duration of edible effects is partly due to the liver converting THC into a more potent and longer-lasting compound, 11-hydroxy-THC.