Researchers from the University of Grenada in Spain have found that smoking cannabis impairs several aspects of vision including contrast sensitivity and ability to focus.
For the study, the researchers recruited 31 cannabis users aged between 19 and 43, of whom 20 were male and 11 female. They tested each person’s visual function both before and after smoking cannabis.
Several traits were measured including static visual acuity (the ability to distinguish details of static objects), contrast sensitivity, night-vision disturbances and pupil size. The participants were also asked whether they perceived their vision worsened or not after smoking cannabis.
All in all, from the visual function tests, the researchers found that smoking cannabis significantly reduced all evaluated aspects of the participants’ vision. This included factors including ability to focus, glare sensitivity, contrast sensitivity, three-dimensional vision and visual acuity.
Curiously, however, 30% of the participants reported that their vision was unaffected by the substance, while 65% reported only slight decreases in visual ability. The researchers noted that the perception of visual ability may be linked to how one’s contrast sensitivity is affected- the more affected it was, the more participants noticed overall visual impairment.
The researchers say that their results indicate a need to increase awareness of how using cannabis may affect vision. This comes especially as cannabis is becoming more and more available, and may affect a person's ability to carry out everyday tasks.
The researchers nevertheless warn that further studies are needed to establish how cannabis really affects vision, especially given the small sample size in their study, and that some of the parameters measured had been measured for the first time. What's more, as each participant smoked cannabis after their normal consumption, they were unable to find how dose and type of cannabis use may affect vision.