Study: Cannabis smoking significantly impairs vision

A study conducted by the University of Granada in Spain indicates that smoking cannabis would significantly impair key visual functions, such as visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus, and sensitivity to glare.

However, more than 90% of consumers believe that cannabis use has no effect on their vision, or only a slight effect.

A group of researchers from the Department of Optics at the University of Granada (UGR) studied the effects of cannabis use on various visual parameters in comparison to the effects that users themselves perceive as affecting their vision.

This study, led by Carolina Ortiz Herrera and Rosario González Anera, has been published in the magazine Scientific Reports. The study’s lead author, Sonia Ortiz Peregrina, explains that cannabis use is on the rise, even though it is an illegal drug. According to the 2019–2020 National Survey on Alcohol, Drugs, and Other Addictions in Spain, cannabis use nationwide has increased since 2011, with 37% of Spanish adults having used cannabis at some point. About 10% have used it in the past year.

In this study, a comprehensive visual test was conducted on 31 cannabis users, both when they had not consumed any substances beforehand and when they were under the influence of cannabis. The researchers also examined the participants’ perceptions of the visual effects of cannabis use.

The results showed that after consumption, visual functions such as visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus, and sensitivity to glare deteriorated significantly. Despite this, not all participants reported a deterioration in their vision after smoking cannabis. In fact, 30 % stated that their vision had not been affected at all, while 65 % reported that it had only slightly deteriorated. The authors note that the visual parameter that may be most strongly linked to users’ perception of visual effects is contrast sensitivity.

The study revealed a negative effect on all visual parameters assessed, with the impact of cannabis on some of these parameters being analyzed for the first time in this research. These results, along with the participants’ lack of awareness regarding the visual impairment caused by smoking cannabis, indicate the need for awareness campaigns, as this visual impairment can pose a danger when performing daily tasks.