Smoking marijuana significantly impairs vision — even though most users disagree

GRANADA, Spain — They say that beauty is in the eye of the beholder. It now appears that vision issues depend on the eye of the smoker. A new study finds smoking marijuana can radically alter key visual functions in the human eye. Despite those results, researchers from the University of Granada say most users don’t believe cannabis is affecting their vision that badly, if at all.

The Spanish study reveals smoking cannabis negatively impacts visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), focus, and glare sensitivity. The findings fly in the face of the popular belief that marijuana may help people with eye diseases like glaucoma. Moreover, researchers say over 90 percent of cannabis users believe smoking the substance won’t impair their vision.

In their experiment, study authors examined the effects of smoking marijuana strictly along visual parameters. They then compared those results to the effect users perceive the drug is having on their sight.

Cannabis users literally don’t see what the problem is

Researchers conducted an exhaustive visual trial with 31 marijuana users. The participants engaged in these tests both while under the influence of cannabis and without the drug in their systems. The team then studied each participant’s perceptions of how smoking cannabis was affecting their vision after use.

Results reveal scores for acuity, contrast sensitivity, stereopsis, focus, and glare sensitivity were all significantly lower after smoking marijuana. Despite this, 30 percent of the group still reported their vision hadn’t suffered at all from smoking. Another 65 percent of the participants said they only noticed a slight drop in their vision due to cannabis use.

So why do so many marijuana smokers perceive their vision to still be good despite findings to the contrary? Researchers say user perception may have a lot to do with contrast sensitivity — which apparently doesn’t translate as well.

Will marijuana legalization lead to more vision impairment?

The Spanish team say their study analyzed marijuana’s impact on certain visual parameters for the first time. Overall, researchers conclude smoking cannabis will negatively all measures of visual health in users.

In Spain, where cannabis is illegal, the study notes use of the drug is still steadily rising since 2011. The country’s national Survey on Alcohol, Drugs and Other Addictions finds 37 percent of Spanish adults have used marijuana at some point.

As more countries, including the United States, legalize marijuana for recreational use, researchers say lack of awareness over how the drug affects vision is a real problem. They add there is an urgent need for campaigns which educate current and potential users on the real effects of cannabis.

“This visual deterioration can pose a danger when performing everyday tasks,” researchers conclude in a university release.

The study appears in the journal Scientific Reports.