When cannabis is consumed, changes in the eyes are often the most immediate and visible physical signs of intoxication. These ocular effects are a direct physiological response to the primary active compound in the plant. Understanding this interaction provides a clear explanation for the characteristic appearance of the eyes.
The Cause and Appearance of Ocular Redness
The most recognizable sign following cannabis consumption is the reddening of the whites of the eyes, often described as “bloodshot” or “cherry eyes.” This distinct appearance is a direct result of the body’s reaction to tetrahydrocannabinol, or THC, the main psychoactive component in cannabis. The redness is not simply due to irritation from smoke, as it occurs even when THC is consumed through edibles or other methods that bypass the lungs.
THC acts as a potent vasodilator, meaning it causes blood vessels throughout the body to relax and widen. When THC binds to cannabinoid receptors in the body, it causes a temporary drop in blood pressure. To compensate for this decrease, the arteries and capillaries expand, or dilate, to allow blood to flow more freely.
This vasodilation is particularly noticeable in the eyes because the capillaries covering the conjunctiva—the clear membrane over the white part of the eye—are small and delicate. As these tiny blood vessels widen, they become engorged with blood and more prominent against the sclera. This creates the tell-tale red appearance, which is often dose-dependent and increases with higher concentrations of THC.
The mechanism that causes ocular redness is also why cannabis has been studied for its potential to treat glaucoma. The vasodilation and subsequent increase in blood flow lead to a temporary reduction in intraocular pressure (IOP), the pressure inside the eye. This effect, however, is short-lived and does not qualify cannabis as a recommended long-term treatment for the condition.
Changes in Pupil Size and Eyelid Appearance
While redness is the primary visual cue, other subtle changes to the eye and surrounding tissue contribute to the overall look associated with cannabis use. The effect of cannabis on pupil size is often less pronounced than the dramatic changes caused by other substances. Some studies suggest acute cannabis use may cause a slight constriction of the pupil, though this effect is variable depending on the individual and the specific compounds consumed.
Other research suggests cannabis can cause pupil dilation, but the consensus is that any change is less dramatic than the massive dilation seen with stimulant drugs. More consistently observed is a change in the pupil’s dynamic response, where the speed at which the pupil constricts and re-dilates in reaction to light is slowed following consumption. This reduced reactivity can be a more reliable indicator of recent use than resting pupil size alone.
The surrounding eyelids also exhibit a characteristic change, often appearing “droopy” or heavy. This presentation is primarily attributed to the muscle relaxation properties of cannabis, which stem from its general depressant effect on the central nervous system. This relaxation extends to the muscles controlling the eyelids, leading to a slight lowering of the upper lid, known as ptosis. Combined with the redness and a potential reduction in blinking, the overall gaze can appear glazed or unfocused.
Duration of Effects and Cosmetic Relief
The visible effects on the eyes are temporary, with the redness subsiding as the body metabolizes the THC. The duration varies significantly based on individual metabolism, the concentration of THC consumed, and the method of ingestion. For most people, the bloodshot appearance typically fades within one to four hours.
When cannabis is inhaled, such as through smoking or vaping, THC enters the bloodstream quickly, leading to a rapid onset of redness that may peak within 30 minutes. Redness from edibles may be delayed, sometimes taking an hour or more to appear. However, it can last longer because the THC is processed more slowly through the digestive system and liver.
For those seeking to mitigate the visible symptoms, over-the-counter eye drops are the most common and immediate solution. These drops contain vasoconstrictor compounds, such as Naphazoline or Tetrahydrozoline, which work by constricting the tiny blood vessels in the conjunctiva. This action directly counteracts the vasodilation caused by THC, temporarily returning the capillaries to their normal size.
Other simple methods, such as ensuring adequate hydration, can help, as dehydration may exacerbate eye dryness and redness. Using a cool compress or moisturizing eye drops can also provide relief by soothing the ocular surface. Choosing strains with a lower THC content is the only way to proactively reduce the intensity of the redness, as THC is the compound responsible for the underlying physiological effect.