Essentially, THC—the psychoactive cannabinoid found in cannabis—is to thank for the infamous red eye effect. Due to how the body reacts to THC once it enters the body, the method of consumption does not directly impact whether or not the reddening of the eyes will ensue. In other words, it doesn't matter if you smoke, vape or eat an edible; once you consume THC, it is a natural occurrence to experience red or bloodshot eyes after the fact.
Once THC enters the circulatory system through the bloodstream, the compound interacts with cannabinoid receptors found throughout the body's endocannabinoid system, including those found in the eyes. Once the cannabinoid binds to the receptor, it induces the dilation or expanding of the blood vessels. This leads to an increase in the blood flow to those areas, causing a measurable decrease in the body's blood pressure and in intraocular pressure. The increase in blood flow to the eye results in the red appearance. However,
according to the American Academy of Ophthalmology, the effects on intraocular pressure are temporary, lasting roughly three or four hours after the consumption of THC. Because of the side effect being temporary, more research is needed to learn how cannabis can potentially aid in treating eye conditions, such as glaucoma.
The increase in blood flow to the eye results in the red appearance.
Glaucoma is an eye condition where the optic nerve becomes damaged over time. In early stages it reduces peripheral vision before possibly leading to blindness. One cause of glaucoma is in higher-than-normal intraocular pressure, which is why researchers have taken a vested interest in learning how cannabinoids could potentially combat the condition. Past research has shown that THC can create a reduction in intraocular pressure, but given the temporary relief, it would reportedly require eight to ten marijuana joints to equal the same effectiveness as the regular eye drops developed for treating glaucoma. On top of that, cannabis can substantially reduce blood pressure and therefore reduce blood flow to the optic nerve, which could increase susceptibility to nerve damage and the worsening of glaucoma. Ultimately, further research can determine exactly how THC could be potentially beneficial in the treatment of glaucoma, to what extent and, most importantly, how to avoid worsening the condition. In the same vein, preliminary research has found that CBD could potentially worsen glaucoma, due to causing an increase in eye pressure, leading healthcare professionals to caution patients accordingly.
While eye redness from consumption typically only lasts for the aforementioned three to four hours, waiting it out is often considered the best remedy, especially considering how the over-usage of eyedrops such as Visine or Clear Eyes could cause increased eye redness, pain, continued redness or irritation. This is because these eye drops are designed to quickly reduce redness, but not to eliminate the underlying cause of the redness in the first place. It may be tempting to reach for these products after cannabis consumption, especially for those who don't want their bloodshot eyes to visibly give away their personal usage, but they are easy to misuse, especially if the medication in the drops wears off before the actual cause does.
Cannabis users can breathe a sigh of relief in knowing that the side effect of red eyes goes away naturally over time and is not considered a major health concern. In fact, the strength of the redness in the eyes directly correlates to the quality and quantity of THC consumed. If you experience noticeably red eyes after consumption, there's a solid chance you've invested in a potent, cannabinoid-rich product, which as many users can attest, is a beautiful thing.