Marijuana Use and Memory
"These results can be interpreted in several ways. A statistically reliable negative effect was observed in the domain of learning and forgetting, suggesting that chronic long-term cannabis use results in a selective memory defect. While the results are compatible with this conclusion, the effect size for both domains was of a very small magnitude. The "real life" impact of such a small and selective effect is questionable. In addition, it is important to note that most users across studies had histories of heavy longterm cannabis consumption. Therefore, these findings are not likely to generalize to more limited administration of cannabis compounds, as would be seen in a medical setting."
Grant, Igor, et al., "Non-Acute (Residual) Neurocognitive Effects Of Cannabis Use: A Meta-Analytic Study," Journal of the International Neuropsychological Society (Cambridge University Press: July 2003), 9, p. 686.