The pupil is the central aperture of the iris, regulating the amount of light reaching the retina. This opening constantly adjusts, constricting in bright light and dilating in darkness. The common belief that cannabis causes pupils to dramatically widen has persisted for decades, often associating the drug with the extreme dilation seen with stimulants. Understanding the true effect of marijuana requires examining the biological interactions between the plant’s compounds and the ocular system.
The Direct Answer: Pupil Size and Cannabis Use
Contrary to popular assumption, studies involving the acute use of cannabis indicate that it does not cause significant dilation, known scientifically as mydriasis. The primary psychoactive compound, delta-9-tetrahydrocannabinol (THC), often results in a neutral effect or, in some controlled settings, a slight constriction (miosis). This response is distinct from the powerful mydriasis caused by stimulants like cocaine or amphetamines, which activate the sympathetic nervous system.
The common misconception likely stems from observing a blunted pupillary light reflex in users. This means the pupil’s ability to constrict quickly and fully in response to bright light is reduced. This sluggish response can make the pupil appear larger than normal in a well-lit environment, contributing to the belief that the pupil is actively widened. Changes in pupil size are strongly correlated with the concentration of THC in the blood.
The Physiology of Pupil Control and Cannabinoid Receptors
The size of the pupil is governed by the autonomic nervous system (ANS), which consists of two opposing branches. The parasympathetic system causes the sphincter pupillae muscle to contract, leading to constriction (miosis). The sympathetic system causes the dilator pupillae muscle to contract, leading to dilation (mydriasis).
THC interacts with the body’s endocannabinoid system by binding to Cannabinoid 1 (CB1) receptors. These receptors are widely distributed throughout the eye, including the retina, ciliary body, and the sphincter pupillae muscle. The presence of CB1 receptors in the sphincter pupillae suggests a direct mechanism for influencing pupil size.
Activation of these receptors by THC appears to shift the balance of the ANS, potentially favoring constriction or, more commonly, dampening the reflex response to light. This dampening effect indicates a change in the speed and extent of the pupil’s adjustment.
Why Cannabis Causes Red Eyes and Reduced Eye Pressure
The most visible ocular effect of cannabis is the reddening of the eyes, which is separate from pupil size changes. This red appearance is caused by vasodilation, the widening of blood vessels throughout the body, including the tiny capillaries on the surface of the eye (the conjunctiva). THC is a potent vasodilator, and as these ocular capillaries expand, they become more visible, creating the characteristic bloodshot look.
This vasodilation also results in a systemic decrease in blood pressure. This mechanism is responsible for the reduction of intraocular pressure (IOP), a medically studied effect of cannabis. THC binds to CB1 receptors located in the ciliary body and trabecular meshwork of the eye.
This interaction decreases the production of aqueous humor, the fluid inside the eye, and increases its outflow, thereby lowering the internal pressure. This pressure-lowering effect is why cannabis was initially studied for treating glaucoma, a condition characterized by abnormally high IOP. The effect is temporary, typically lasting only a few hours.
Factors Influencing Observed Pupil Size After Use
A user’s perception of pupil size is often influenced by external and biological factors. Ambient lighting conditions are the most significant variable, as pupils naturally enlarge in dim light, regardless of cannabis consumption. This natural mechanism can be mistaken for drug-induced dilation.
The chemical composition of the cannabis strain is also a factor, particularly the ratio of THC to cannabidiol (CBD). Furthermore, the co-ingestion of other substances can mask the effect of cannabis. Stimulants, for example, cause powerful mydriasis, which would override the subtle or constrictive effect of THC.