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Home Physiology Why does cannabis make your eyes red?
Dec 22, 2019 2 min read

Why does cannabis make your eyes red?

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by Philip Ghezelbash
A woman wearing sunglasses

Red eyes are a perfectly normal and harmless side effect of using cannabis. When marijuana is inhaled or ingested, it lowers your blood pressure, causing blood vessels and capillaries to widen.

When blood vessels and capillaries widen, more blood is able to flow around the eyes and this is what causes redness.

In general, the more marijuana you consume, the redder your eyes will become.

It’s not the smoke

THC is the cannabinoid in cannabis responsible for the high experienced by users. THC is also the main cannabinoid in marijuana that causes blood vessels in the eye to widen, and therefore the eyes to redden. 

Strains of marijuana with less THC may not cause red eyes, while a strain of cannabis very high in THC may result in noticeable redness of the eyes. 

One common misconception is that smoke from marijuana causes red eyes. We know this isn’t true as edibles also cause redness in the eyes. 

It’s important to note that while it’s not marijuana smoke that causes the red-eye effect, there are some people who are susceptible to smoke allergies, and who may also experience eye redness from tobacco smoke, air pollution, and incense. 

Consuming edibles may be a good alternative for anyone with a smoke allergy, as they won’t provoke irritation.

How long do red eyes last?

According to a report by the American Academy of Ophthalmology… 

“Research in the 1970s and 1980s did show a measurable decrease in intraocular [eye] pressure for three or four hours after smoking cannabis or ingesting THC as a pill or injection.”

Based on this report, eye redness from cannabis use should only last four hours at the most. However, it’s well known that the psychotropic effects from ingesting cannabis as an edible can last up to six hours or even longer. 

Therefore, your eyes may remain slightly red for longer than four hours if you consumed a cannabis edible, or if your eyes are particularly sensitive.

How can you decrease eye redness? 

Besides waiting for the redness to subside naturally, there are some measures you can take to potentially decrease it.

  • Use cannabis strains containing less THC or use less cannabis overall.
  • Drink more fluids in order to decrease dryness.
  • Try eye drops to decrease dryness. There are also eye drops specifically formulated for red eyes which can be found at most convenience stores and pharmacies. 
  • If all else fails, sunglasses can be a good way to conceal your red eyes.

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