The small study in France found prolonged marijuana use could wear down retinal ganglion cells, the nerves that sit just behind the retina. A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye.
It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. These cells are key for transmitting information back to the brain in order to form an image.
The study found people who smoked weed for around six years experienced a delay in neural signals to the brain – even after giving up the drug. The finding complicates scientific understanding on the link between cannabis and vision, since previous studies have shown the drug helps patients with glaucoma, and even improves night vision.
Reacting to the development, Lead Investigator, Dr. Vincent Laprevote, of the Pole Hospitalo-Universitaire de Psychiatrie du Grand Nancy in France, said: “Our findings may be important from a public health perspective since they could highlight the neurotoxic effects of cannabis use on the central nervous system as a result of how it affects retinal processing.
“Independent of debates about its legalisation, it is necessary to gain more knowledge about the different effects of cannabis so that the public can be informed.
“Future studies may shed light on the potential consequences of these retinal dysfunctions for visual cortical processing and whether these dysfunctions are permanent or disappear after cannabis withdrawal.”
The small study involved 52 participants – 28 cannabis users, 24 never users. All of the cannabis users stopped consuming the drug before the trial started, but had been regularly using it for at least six years.
The researchers used a standard electrophysiological measurement called pattern electroretinography (PERG), which involved averaging a high number of responses.
After adjustment for the number of years of education and alcohol use, there was a significant increase for cannabis users of the amount of time it took for the retina to transmit signals to the brain.
There are many known effects on the body when using marijuana. Some of these relate to the eyes while others impact other areas of the body. Studies have found that attention, memory and learning are all impaired by heavy marijuana use, even after stopping the drug for more than 24 hours. Marijuana users also exhibit impaired driving abilities, slower reaction times and difficulty concentrating. Marijuana can also impact perception, making colors appear brighter, etc. When it comes to eyesight marijuana commonly alters peripheral vision.
