This is the opening of a submission by Dr Stuart Reece to the FDA relating to the re-scheduling of cannabis:

“I am very concerned about the potential for increased cannabis availability in USA implied by full drug legalization; however, a comprehensive and authoritative submission of the evidence would take weeks and months to prepare. Knowing what we know now and indeed, what has been available in the scientific literature for a growing number of years concerning a myriad of harmful effects of marijuana, marijuana containing THC should not be reclassified. These effects that are now well documented in the scientific literature include, alarmingly, harm involving reproductive function and birth anomalies as a result of exposure to or use of marijuana with THC.

In addition to all of the usual concerns which you will have heard from many sources including the following I have further particular concerns:
1) Effect on developing brains
2) Effect on driving
3) Effect as a Gateway drug to other drug use including the opioid epidemic
4) Effect on developmental trajectory and failure to attain normal adult goals(stable relationship, work, education)
5) Effect on IQ and IQ regression
6) Effect to increase numerous psychiatric and psychological disorders
7) Effect on respiratory system
8) Effect on reproductive system
9) Effect in relation to immunity and immunosuppression
10) Effect of now very concentrated forms of cannabis, THC and CBD which are widely available
11) Outdated epidemiological studies which apply only to the era before cannabis became so potent and so concentrated 

These issues are all well covered by a rich recent literature including reviews from such major international authorities as Dr Nora Volkow Director of NIDA at NIH, Professor Wayne Hall and others “

The full text can be read here

Source: Letter from Dr Stuart Reece to FDA April 2018