It's official. Smoking cannabis substantially alters the way you see

But you might not notice.
Derya Ozdemir

A team of researchers from the University of Granada (UGR) has found that smoking cannabis drastically alters vision, according to a study published in the journal Scientific Reports.

While a shockingly high number of cannabis users claimed to be able to see just fine, the new study has found the drug does the complete opposite by altering key visual functions like visual acuity, contrast sensitivity, the ability to focus, and more.

This highlights the need to carry out public awareness campaigns about the visual impacts of cannabis, as the visual deterioration can pose a danger when performing daily tasks such as driving.

Your eyes on cannabis 

One of the most well-known and noticeable effects of smoking marijuana is the red eyes. This happens because THC, or tetrahydrocannabinol, the chemical responsible for most of marijuana's psychological effects, lowers blood pressure, dilates the blood vessels, and enhances blood flow throughout the body. This process causes the blood vessels in the eyes to dilate, resulting in redness or bloodshot eyes. However, the impact of marijuana on peripheral vision, changing eye pressure, and visual processing is still not completely understood, and further research is needed in this area.

In a bid to do just that, researchers from the University of Georgia's Department of Optics conducted an extensive visual trial on 31 cannabis users, both before and after they had consumed any substance, per a press release. They also investigated the subjects' perceptions of the visual impacts of the drug.

It was seen that the subjects' visual features such as visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus, and glare sensitivity significantly worsened following consumption. 

Despite all that, 65 percent of the participants responded that their vision had worsened only slightly, while 30 percent stated it had not worsened at all. Contrast sensitivity is the visual attribute that the authors believe is most strongly linked to consumers' experience of the visual effect.

Altering key functions like three-dimensional vision

With the effect of cannabis on some of the parameters being studied for the first time in this study, the researchers discovered that cannabis had a detrimental effect on all of the visual parameters they assessed. Still, it should be noted that the study is based on only 31 volunteers, and a much larger version of the experiment should be conducted to further validate the findings. 

Another study that investigated the effects of smoking cannabis on vision and driving performance also found that the visual effects of cannabis could impact driving performance, jeopardizing driving safety. Overall, it appears that information and awareness initiatives are critical for reducing the prevalence of cannabis-impaired driving.

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