Rationale: Cannabis is known to impair cognition, attention, and memory. This study investigated effects of long term cannabis use in reading comprehension. 

Method: Subjects were 20 chronic cannabis users, and 20 non-cannabis users, all confirmed to be drug free for the previous 24 hours. Participants were asked to read a set of 96 declarative sentences, each containing a target word, while their eye movement was tracked. The target words varied in terms of length and how common they are in the English language. Researchers measured how long it took people to read the sentences, how many times they had to re-read particular words, and how long they spent looking at specific words. They also assessed participants’ reading comprehension.

Results: Reading speed and comprehension were impaired in cannabis users compared to nonusers. While the cannabis group read more slowly, as indicated by average amount of time spent reading each sentence (about 3.5 seconds vs. about 3.2 seconds) and were more likely to reread text (26% vs. 18%) their comprehension was lower than nonusers (79.4% vs 87.1%). Deficits in cannabis users were especially pronounced when reading more complex sentences.

Meaning: Regular cannabis users have deficits in reading comprehension  despite the fact that they read more slowly. This study adds to a growing body of evidence suggesting cognitive impairments in regular cannabis users.