Just last year, President Obama came out saying that Pot is no more dangerous than Alcohol. I believe this is the first time a sitting president has come out for the drug Marijuana.
Directly after this interview, the director of the NIDA came out and rebuked President Obama saying that it is harmful and has long term affects on the brain based on a recent large scale study in 2013.
To quote some of the report.
"Participants who used cannabis heavily in their teens and continued through adulthood showed a significant drop in IQ between the ages of 13 and 38—an average of 8 points for those who met criteria for cannabis dependence. (For context, a loss of 8 IQ points could drop a person of average intelligence into the lowest third of the intelligence range.)"
Another report from Brain was released in 2012 with long term brain defects as associated with Marijuana.
"Axonal connectivity was found to be impaired in the right fimbria of the hippocampus (fornix), splenium of the corpus callosum and commissural fibres. Radial and axial diffusivity in these pathways were associated with the age at which regular cannabis use commenced. Our findings indicate long-term cannabis use is hazardous to the white matter of the developing brain."