The effects on visual physiology of cannabis with high content of the psychotropic tetrahydrocannabinol (THC) have been little studied.  There is a lack of data related to cannabis use and visual impairment for parameters like depth perception, night-vision performance, visual acuity or Mandelbaum effect that is when the eye changes its fixation from a distant point in the space to a nearby object.  In a recent research the effects of recreational use cannabis on visual function and self-perceived visual quality were analysed on thirty-one cannabis users.  The participants were 20 males and 11 females selected between 19 and 43 years old, all occasional cannabis users and without hystory of alcohol or drug disorders. They were asked to avoid cannabis four days before the tests and to abstain from alcohol 24 hours prior the study.  What came out from the study is that cannabis worsen static visual acuity, contrast sensitivity, three-dimensional vision and increased accomodative lag, that is the ability to adjust the eye to near and far targets. In addition to these data also night-vision was altered and halos and greater glare were observed within the participants to the study. The research investigated also the correlation between objective changes in visual function and the users own subjective perception. It was noticed that two thirds of the sample participants believed their vision was impaired by smoking cannabis and interestingly the group of them thinking that the substance was not affecting their vision did use cannabis more frequently. The parameter of contrast sensitivity was found objectively worse in people who subjectively indicated that their vision was unpaired by cannabis. 
An important aspect to take into consideration is that the blood concentration of THC was not measured, thus it was not possible to relate this parameter to the effects. Moreover, the relatively small sample size didn’t allow to establish groups from the consumption profile perspective in order to assess the subjective perception of cannabis effects. For this reason, further studies should be done to compare in depth the correlation between self-perceived and objective impact of cannabis on visual performance. The inclusion of pharmacological, imaging tests or electrophysiological studies to the research could support the findings linking the neuronal and signaling along the visual pathway with the functional effects. 
References: Sonia Ortiz-Peregrina;Carolina Ortiz;Miriam Casares-López;José R. Jiménez;Rosario G. Anera; (2021). Effects of cannabis on visual function and self-perceived visual quality . Scientific Reports, (), –.doi:10.1038/s41598-021-81070-5 [Journal impact factor = 4.996] [Times cited = 11]
 Alsaqr AM, AlShareef H, Alhajri F, Abusharha A, Fagehi R, Alharbi A, Alanazi S. Accommodative Response in Patients with Central Field Loss: A Matched Case-Control Study. Vision (Basel). 2021 Jul 9;5(3):35. doi: 10.3390/vision5030035. [Journal impact factor = 1.039] [Times cited = 1]
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