Smoking cannabis significantly impairs vision, yet more than 90% of users believe it has no or very little effect on their vision, a new study has found.  

With cannabis use on the rise in Spain, researchers at the University of Granada decided to study the effects of cannabis smoking on various visual parameters and compare that to the effect the users themselves perceived. A visual trial was conducted on 31 cannabis users, both under the influence and not. Following consumption visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus and glare sensitivity all significantly worsened. Despite this, not all subjects reported a worsening of their vision after smoking cannabis. In fact 30% reported their vision had not suffered at all, while 65% responded it had worsened only slightly. The researchers also noted the visual parameter most strongly linked to users' perception was contrast sensitivity. 

The study was published in Scientific Reports.