“It’s for my glaucoma, maaaan”. Stoners everywhere have perpetuated all kinds of health half-truths about marijuana; it cures glaucoma, it cures cancer, it gives you super night vision, it lets you see through time. Part of the reason for the perpetuation of these apocryphal claims is the lack of data surrounding marijuana use; that may change quite rapidly, with Canada’s legalization of marijuana now complete. Studies can now take place more easily, but in the interim, it’s important to highlight what we do know (very little) and what we don’t know (a lot) about marijuana’s effects on your eyes.

First thing is first; the dreaded red-eye. Some of you may still have memories of your parents calling you out for eyes as red as the devil is, with unexplained, sudden, and otherwise symptomless seasonal allergies being the explanation. Marijuana causes red eye because THC, one of the active compounds in marijuana, dilates blood vessels and increases blood flow through the body; the eyes blood vessels expand too, and because blood vessels are red, your eyes look red too!

This same physiological change is why some folks claim marijuana cures glaucoma. So far as we know, this is not true. The increased blood flow and dilated vessels can reduce intraocular pressure, which is the primary cause of glaucoma, but only for a short amount of time; three to four hours. Other procedures to treat glaucoma, including eye drops and surgery, are much more effective at treating the disease. Additionally, if a doctor of any stripe is advising you to smoke something, you should start asking a lot of questions; smoking is bad for your health because it’s bad for your lungs. Should you be prescribed marijuana to treat any kind of illness, it’s probably best to get your THC or CBD in pill form.

There are two other studies, both with too-small sample sizes to be conclusive, that have something to say about marijuana and your eyes. One is based on tales of far-seeing Jamaican fisherman with incredible night vision and prodigious cannabis consumption; to test this theory, cannabinoid eye drops were put on tadpoles eyes. The cannabinoids made certain retinal cells more light-sensitive; tadpoles aren’t people though, so more evidence is needed. In another study, there were 28 marijuana users and 24 non-users; the users had a delay of about 10 milliseconds for electrical impulses in their retinal ganglion nerves. This could mean that marijuana slows visual processing, but the sample size is quite small; again, more research is needed.

So, if you enjoy marijuana, feel free to celebrate legalization, but if anyone tries to tell you “weed does X for your eyes!”, treat the claim with skepticism; we have very little data on the drug, and using it for medicinal purposes without consulting your optometrist is a bad idea. The best thing you can do for your eyes while we wait for those studies? Get an eye exam done regularly to preserve your vision, so you can see all shades of green.