Marijuana and IQ:
'“Scientifically, these are extremely preliminary findings,” cautions Carl Hart, associate professor of psychology at Columbia University, who has studied the cognitive effects of marijuana in humans in the lab and was not associated with the research. [...] There are also other factors — such as child abuse or other trauma — that might lead people to seek escape in heavy marijuana use and could also affect brain function. Meier and her colleagues did not examine these factors but say it’s possible that such elements could explain the results better than marijuana itself.' - http://healthland.time.com/2012/08/28/does-weekly-marijuana-use-by-teens-really-cause-a-drop-in-iq/
Alcohol:
"researchers confirmed previous findings that alcoholism is associated with thinking problems and lower IQ" - http://www.ur.umich.edu/0506/Oct17_05/15.shtml
"heavy drinking may have extensive and far–reaching effects on the brain, ranging from simple “slips” in memory to permanent and debilitating conditions that require lifetime custodial care." - http://pubs.niaaa.nih.gov/publications/aa63/aa63.htm
and is not carcinogenic.
"Alcohol is a known cause of cancers of the:
Mouth
Throat (pharynx)
Voice box (larynx)
Esophagus
Liver
Colon and rectum
Breast
"Alcohol may also increase the risk of cancer of the pancreas." - http://www.cancer.org/cancer/cancercauses/dietandphysicalactivity/alcohol-use-and-cancer
Also there are clear standards for driving under the influence of alcohol that cannot be so clear for marijuana.
Roadside sobriety tests can fill the gap until we get biochemical tests in place.
If marijuana is so easy to obtain for teen agers, it is even easier for adults. So, what’s the big deal?
The big deal is that keeping marijuana selling illegal hyperinflates its profits and channels those profits into criminal hands.
I know that the Gateway drug claim is controversial
The gateway claim is nonsense - research shows that the correlation between earlier marijuana use and later use of other drugs can be explained by a "common-factor" model, that is, a third factor that causes both results, such as individuals' opportunities and unique propensities to use drugs, or mor broadly a social or psychological predisposition towards anti-social behaviour. (http://www.rand.org/pubs/research_briefs/RB6010/index1.html, http://rds.homeoffice.gov.uk/rds/pdfs2/hors253.pdf)
The study was done very carefully over a period of 30 or more years and included a group, who did not use marijuana. The Universities that were involved were top notch.
The gateway claim is nonsense - research shows that the correlation between earlier marijuana use and later use of other drugs can be explained by a "common-factor" model, that is, a third factor that causes both results, such as individuals' opportunities and unique propensities to use drugs, or mor broadly a social or psychological predisposition towards anti-social behaviour. (http://www.rand.org/pubs/research_briefs/RB6010/index1.html, http://rds.homeoffice.gov.uk/rds/pdfs2/hors253.pdf)
I see I forgot to mention that the same sort of correlation between earlier marijuana and later harder drugs also exists between earlier alcohol and tobacco and later illegal drugs - so if marijuana is a "gateway" so are alcohol and tobacco.