According to this post at the Washington Post, people who smoked pot regularly for at least five years had some (slight?) short-term memory problems in middle age, compared with those who didn't. But the posting also notes some caveats concerning the study:
One important caveat is that a study like this can't determine causality. It could be the case that heavy pot use makes your short-term memory bad, or it could be that people who operate at a lower level of cognitive function are more inclined to use marijuana heavily.
It's also worth noting that the other cognitive abilities researchers tested -- focus and processing speed -- did not seem to be significantly impacted by heavy marijuana use.
The association between short-term memory declines -- potentially permanent ones -- and heavy pot use is very real, according to this study, and shouldn't be discounted. On the other hand, it's also quite surprising that you can smoke weed literally every single day for five years, and not have it impact your problem-solving abilities or your ability to focus at all. [emphasis added] These findings also need to be understood in relation to what we know about the severe cognitive effects of persistent, heavy alcohol use, which include irreversible brain damage.
I would add another caveat: The presumption in the article is that the heavy marijuana users smoked it. What if, instead, people ingested it? There would be less damage to the lungs and less direct effect from shortages of oxygen to the brain.
Now, if only recreational use of marijuana were completely legalized, ....