Attention/concentration: Marijuana impairs light users’ attention and concentration but doesn’t appear to affect regular or heavy users within six hours of smoking or ingesting it. In the long run, researchers have found that after 3 weeks or more since last using marijuana, subjects’ attention and concentration return to normal. “In five of seven studies, no attention or concentration impairments were found in subjects who had remained abstinent from 28 days to one year (Lyons et al., 2004; Pope et al., 2001; 2002; 2003; Verdejo-Garcia et al., 2005).” The other two studies found differences in attention and concentration between heavy and non-users after 28 days, but the reviewers note that the disparate findings could be due to measuring different types of processing skills.
Working memory: Several studies likewise found no residual or long-term effects on working memory. A 2002 study, for example, tested 77 heavy smokers for days after abstaining from smoking pot. Memory impairment was found for heavy users up to 7 days after using marijuana, but by day 28 their memory test results didn’t differ significantly from control subjects. In other words, even if your memory is affected when you smoke up, after you stop it will likely go back to normal with time.