The phenomenon of bloodshot or red eyes, medically known as conjunctival injection, is a recognizable physical sign following cannabis consumption. This visible effect is a predictable physiological response linked to the primary psychoactive compound, delta-9-tetrahydrocannabinol (THC). The redness is a systemic effect, meaning it happens regardless of whether cannabis is smoked, vaped, or consumed as an edible, signaling a change in the body’s cardiovascular system. This reaction is a temporary side effect of THC’s interaction with the body’s regulatory systems.
The Action of THC on Blood Pressure
The mechanism causing red eyes begins with THC’s action on the circulatory system and blood pressure. When THC enters the bloodstream, it interacts with the endocannabinoid system, binding to CB1 and CB2 cannabinoid receptors found in tissues like the heart and blood vessels. This interaction triggers effects that cause the smooth muscle cells lining the blood vessels to relax.
The relaxation of these muscle cells leads to vasodilation, the widening of blood vessels throughout the body. Systemically, this widespread vasodilation results in a temporary drop in overall blood pressure, known as hypotension. The cardiovascular system often attempts to compensate for this drop by increasing the heart rate, especially with higher doses. This drop in pressure is the precursor for the visible changes in the eyes.
The Mechanism Causing Ocular Redness
The red-eye appearance is a direct, localized consequence of the systemic blood pressure drop induced by THC. Since blood vessels are widened throughout the body, the tiny capillaries in the eyes also expand. These vessels are particularly noticeable in the conjunctiva, the clear membrane covering the white part of the eye. The increased diameter of these minute vessels allows a greater volume of blood to flow through them.
This surge of blood flow into the expanded capillaries makes the vessels significantly more visible against the white background of the eye, resulting in the characteristic “bloodshot” coloration. This confirms the redness is not caused by smoke irritation, as the same reaction occurs when THC is ingested in edible form. The vasodilation in the eye also slightly lowers intraocular pressure. This pressure reduction mechanism has led to research into cannabis for conditions like glaucoma.
How Long Redness Lasts and Safety
The duration of ocular redness is temporary, correlating with the peak concentration and half-life of THC in the body. For most individuals, the red-eye effect typically lasts between one to two hours, but can persist for up to three or four hours with high-potency products or edibles. The exact timeline depends on metabolism, the dose consumed, and the method of ingestion; smoking or vaping leads to a quicker onset and offset compared to edibles.
From a health standpoint, the redness is generally considered a harmless, short-term side effect of THC-induced vasodilation. It is not an indication of damage or long-term risk to ocular health. For those who wish to mitigate the visible effect, over-the-counter eye drops containing vasoconstrictors are a common method. These drops work by locally reversing the vasodilation, causing the blood vessels in the eye to constrict and reduce the visible redness.