Does Marijuana Use Affect a Person's Vision?

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Regular marijuana use may affect how well certain cells in the eye's retina function, a small new study finds.

But some experts say that the evidence presented in the study isn't strong enough to support the link between these two factors. The cells that the researchers focused on in the study, called retinal ganglion cells, are located near the inner surface of the eye's retina. These cells collect visual information and transmit it to the brain.

The study included 52 people who had used marijuana at least 7 times per week during the previous month and 24 people who had never used marijuana. The people in both groups were between 18 and 35 years old. The researchers verified the marijuana use by testing the people's urine for THC, marijuana's main psychoactive ingredient. [7 Ways Marijuana May Affect the Brain]

The researchers tested the participants' vision and found that their eyesight was relatively good, and that no one in the study group reported having any visual problems from using marijuana such as blurred vision, according to the study, which was published today (Dec. 8) in the journal JAMA Ophthalmology.

To study how well the participants' retinal ganglion cells worked, the researchers used a method called pattern electroretinography, which provides information about how well those cells function, as well as how fast they transmit visual information from the retina to the brain.

The test revealed that, compared to the people who didn't use marijuana, those who did use the drug had a slight delay in how long it took for information to be transmitted from the retina to the brain, according to the study.

It's not clear whether this potential effect of marijuana is permanent, or would stop when a person stops using the drug, said study co-author Dr. Vincent Laprévote, a psychiatrist at Pôle Hospitalo-Universitaire de Psychiatrie du Grand Nancy in France.

But some experts argue that the study had significant limitations, and because of this, it's unclear whether or not there is an actual link between marijuana use and these effects.

More research is needed to determine whether marijuana use really is linked to changes in the functioning of those cells, said Dr. Christopher J. Lyons, an ophthalmologist at the University of British Columbia, who was not involved in the study. [Marijuana Could Treat These 5 Conditions]

Lyons noted that although the electroretinography results suggested a difference between marijuana users and nonusers, the delay didn't seem to translate into actual problems with the users' vision.

Indeed, the marijuana users in the study did not experience any actual visual symptoms or changes in the quality of their vision, Lyons told Live Science.

In an editorial that was published in the same journal as the study, Lyons and Dr. Anthony Robson, an ophthalmologist at the University College London who was also not involved in the study, noted that the researchers examined the people's marijuana use through urine tests, which are not as accurate as blood tests.

In addition, there are many other factors such as tobacco use, diet and lifestyle that might affect the functioning of a person's retinal cells, and these factors could have affected the results of the study, Lyons and Robson wrote.

Moreover, while the study authors said that the abnormalities that they found in the study that involved the functioning of the marijuana users' retinal ganglion cells might explain why some marijuana users experience altered vision, they did not present any evidence in the study to support this statement.

Originally published on Live Science.