Health concerns regarding marijuana tend to come from a self-fueling group of discredited scientists funded by the pharmaceutical, prison, tobacco, and alcohol industries. They push non-peer-reviewed papers, fraught with conjecture and confounding variables, while relying upon reports issued by others in their own group to further support their own grossly misleading research and clearly biased agendas.

The Duke University (New Zealand) study, the one which claimed that smoking marijuana in your teens leads to a long-term drop in IQ, has since been utterly rebuked by a new paper, published by the Proceedings of the National Academy of Sciences that examined the research and found its methodology to be flawed.

"...existing research suggests an alternative confounding model based on time-varying effects of socioeconomic status on IQ. A simulation of the confounding model reproduces the reported associations from the [August 2012 study], suggesting that the causal effects estimated in Meier et al. are likely to be overestimates, and that the true effect could be zero".
—Ole Rogeberg.

Source: www.salon.com/.../

Here is a recent peer-reviewed Study proving that Marijuana is not linked with Long Term Cognitive Impairment:

Amy M. Schreiner of the Department of Psychology at the University of Central Florida recently led a study that looked at 33 existing meta-analyses of cognitive impairment experienced by heavy cannabis users. Schreiner was unable to provide evidence of long-lasting impairment. Specifically, the participants demonstrated no significant cognitive deficiencies once the intoxication period ended. Additionally, Schreiner found no symptoms of impairment in the individuals who had abstained for 25 days. In conclusion she said, "These results fail to support the idea that heavy cannabis use may result in long-term, persistent effects on neuropsychological functioning."

Reference:

Schreiner, A. M., Dunn, M. E. (2012). residual effects of cannabis use on neurocognitive performance after prolonged abstinence: A meta-analysis. Experimental and Clinical Psychopharmacology. Advance online publication. doi: 10.1037/a0029117