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Why Does Weed Make Your Eyes So Bloodshot?

Whether you smoke it, eat it or suck it in through a vape, cannabis makes you look like you’ve just been chopping onions. Why, cruel god? Why?

You can always tell a stoner by their glazed, red eyeballs, a side effect of the potent skunk coursing through their system. But while some brave cannabis lovers display their bloodshot stare with pride, most would rather float under the radar.

So, what gives? Why does weed make your eyes red, anyway?

According to chemist Tony Ferrari, chief science officer at SunMed CBD, this is due to “increased blood flow to the eye area and is a symptom of psychoactivity and [THC] binding to the CB1 receptor.” CB1 receptors, which regulate the release of feel-good neurotransmitters like serotonin, dopamine and glutamate, are especially concentrated in and around the eyes, and when THC binds to them, it dilates nearby blood vessels, increasing blood flow. As a result, the brimming blood vessels in your eyes make them look red.

This excess blood flow decreases your overall blood pressure, which also explains why weed can sometimes make you feel dizzy. Interestingly, though, that reduced blood pressure makes THC a potentially viable treatment for glaucoma, a group of eye conditions often caused by increased eye pressure, which can result from high blood pressure. (Weirdly, however, CBD has been shown to increase eye pressure.)

How red your eyes become doesn’t depend on your ingestion method — i.e., edibles turn them crimson, too. However, the more THC you consume, the redder your eyeballs will become.

As for how to hide the redness, your best bets are eyedrops or good old-fashioned sunglasses. Alternatively, you could always blame onions.