Does Cannabis Affect Vision? 

Smoking weed can temporarily make your eye sight slightly worse, a new study has found.

Researchers at the University of Granada carried out a study which found that cannabis alters key visual functions. 

These functions include; contrast sensitivity, visual acuity, three-dimensional vision, glare sensitivity, and the ability to focus. 

The effects seem to be mild, however – not enough to fail standard eye tests.  

The study, published in the Journal Scientific Reports, observed participants’ vision during a series of seven standard vision tests before and after smoking cannabis. 


31 adult participants performed the seven vision tests once without cannabis, and then once more 20 minutes after smoking a joint.  

Researchers started by measuring participants’ visual acuity. The task was to identify different-sized letters on a standard eye chart. 

Then they tested contrast sensitivity, and the ability to recognise patterns with varied light and dark contrasts.  

Next up was the three-dimensional vision test, which is also known as the stereoacuity test. Participants wore a pair of 3D glasses and were asked to identify dots on the screen that appeared to have a three-dimensional shape. 

Other visual functions that were tested included participants’ ability to bring nearby and distant objects into focus (the accommodation test), and also their susceptibility to glare. 

This was also the first time that research has been done on the effects of cannabis on many of the visual functions.  


Authors of the study found consistent deficits in all of the visual functions after the consumption of cannabis.  

However, the differences weren’t huge.  

There was still a large portion of participants (30%) who hadn’t reported any decrease in their vision after smoking a joint. 

The 65% of people who had reported a decrease in their vision said it was only slightly.  

Although cannabis negatively impacted all of the different tests, authors noted that the performance would still be more than sufficient to pass standard eye tests. 

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