A common effect following cannabis use is the physical change in the eyes, which often appear heavy-lidded and red. This is a direct physiological response to the active compounds in the substance. The characteristic “low” look and ocular redness result from complex interactions within the nervous system and vascular tissue. This article details the specific mechanisms that cause the eyelids to droop and the eyes to become visibly bloodshot.
The Primary Mechanism of Eyelid Droopiness
The appearance of “low” or droopy eyelids is medically referred to as ptosis, and in this context, it is a consequence of muscle relaxation. The muscles responsible for keeping the upper eyelid fully raised, particularly the levator palpebrae superioris muscle, lose some of their normal tension. This loss of tone is a direct result of the substance’s effect on the autonomic nervous system.
The sympathetic nervous system, often called the “fight or flight” response, maintains a constant level of activity that contributes to muscle tone. When active compounds like delta-9-tetrahydrocannabinol (THC) enter the system, they interfere with this sympathetic output. Studies show that acute cannabis use reduces muscle sympathetic nerve activity in humans.
This reduction in sympathetic tone leads to a decrease in muscle contraction, including in the muscles controlling the eyelid. The resulting partial relaxation of the levator muscle group causes the characteristic resting droop, sometimes described as pseudoptosis.
Understanding Ocular Redness
The visible redness in the eyes, commonly known as “bloodshot eyes,” is caused by a distinct vascular change called vasodilation. Vasodilation is the widening or expansion of blood vessels, and in the eye, it specifically affects the tiny capillaries in the conjunctiva, the clear membrane covering the white part of the eye. Tetrahydrocannabinol (THC) is the primary compound responsible for triggering this expansion.
When these capillaries widen, blood flow to the eyes increases, making the vessels more visible against the white sclera. This effect is not limited to the eyes, as THC causes vasodilation throughout the body, which can also contribute to a temporary decrease in blood pressure. This increase in blood flow is directly linked to the substance’s ability to reduce intraocular pressure (IOP), which is why cannabis has been studied for conditions like glaucoma.
The degree of ocular redness is often correlated with the dose of THC consumed and is a temporary side effect of this vascular expansion. The redness occurs regardless of the method of consumption, meaning it is a systemic effect of the chemical and not merely an irritation from smoke.
How the Body Processes the Change
The physiological changes observed in the eyes are initiated by the interaction of the active compounds with the body’s own communication network, the Endocannabinoid System (ECS). The ECS is a vast system of receptors and signaling molecules that regulate numerous functions, including mood, pain sensation, and autonomic function.
The primary psychoactive component, THC, acts as an external mimic of the body’s natural endocannabinoids. It achieves its effects by binding to specific receptors, primarily Cannabinoid Receptor 1 (CB1), which are distributed throughout the central nervous system, vascular tissue, and the eyes themselves.
This specific receptor binding triggers the cascade of effects. In the nervous system, CB1 activation reduces sympathetic output, causing eyelid droopiness. Simultaneously, CB1 receptors in the smooth muscle of blood vessels are activated, leading to the vasodilation that causes ocular redness.