Those who use cannabis (as opposed to having used it in the past) may be more likely to have short-term memory problems.
Long-term use of marijuana has been found to affect verbal fluency, verbal memory, attention, and psychomotor speed, with longer use correlating with greater impairment. However, although there have been a number of studies into the long-term effects of cannabis of memory and learning, most are small or not sufficiently rigorous. We cannot, therefore, make any clear conclusions on this topic. Verbal memory appears to be the specific type of memory that is particularly vulnerable, but there is no clear evidence of long-term damage.
It may be that the effects of cannabis on cognition depends on various factors. For example, there is some evidence that starting in adolescence may be more likely to produce impairment, some suggestion that it depends on the strain of cannabis (there’s some evidence that cannabidiol can counteract the memory-impairing effects of the psychoactive component THC), a possibility that certain conditions could interact (one study found that MS sufferers who used marijuana for pain relief performed significantly worse on tests of attention, speed of thinking, executive function and visual perception of spatial relationships between objects).