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Smoking marijuana can seriously impair consumers' vision, study finds

The majority of cannabis users don’t recognize these vision impairments

Photo (c) Anton Petrus - Getty Images
Many consumers have started using marijuana to treat a number of medical conditions, including blood pressure, chronic pain, and mental health concerns. 

Now, a new study conducted by researchers from the University of Granada found that smoking marijuana may lead to vision impairments. Though the majority of cannabis users don’t recognize any changes to their vision, the researchers found that the negative effects can make it harder to perform day-to-day tasks.

“This study shows that smoking cannabis has significant adverse effects on certain visual functions, including visual acuity and contrast sensitivity, as well as in nighttime-related visual parameters such as the [visual-disturbance index] and intraocular straylight,” the researchers wrote

Understanding visual impairments

To understand what effect cannabis can have on consumers’ vision, the researchers observed 31 participants’ visual acuity both when they had and hadn’t smoked it. The researchers tested the participants’ night vision, contrast sensitivity, visual acuity, accommodative response, and pupil size in both trials. Additionally, participants reported on how they believed their vision was affected both with and without the drug. 

Ultimately, the researchers observed significant changes to every aspect of the participants’ vision after smoking cannabis. Some notable changes included increased susceptibility to visual glares, issues with depth perception, loss of general visual acuity, and a decreased ability to focus. 

It’s important to note that nighttime vision was also seriously compromised. The team noted that cannabis impacted the participants’ visual disturbance, which is when short flashes of light momentarily impair vision. Additionally, the participants had issues with contrast sensitivity, or the ability to pick out fine shades of lightness or darkness. 

In evaluating the participants’ perceptions of their own visual abilities both pre- and post-cannabis, the researchers learned that there was a significant contrast between actual vision and perceived vision. More than 65 percent of the participants believed they had just a slight change in vision after smoking, while 30 percent believed there was no vision change due to cannabis. 

Moving forward, the researchers hope that these findings highlight just how much consumers’ vision can be affected by cannabis and what this can mean as they go about their daily routines. 

“Our results could help generate a better understanding of the visual changes related to cannabis use and their implications for everyday tasks, raising awareness among users of the risks involved in consuming this drug,” the team wrote. 

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