The following are citations from Drug Enforcement Administration. Denial of Petition To Initiate Proceedings To Reschedule Marijuana. (Federal Register. August 2016; 53687-53766. 81 FR 53687.):

Cannabis is not a gateway drug:

• “Overall, research does not support a direct causal relationship between regular cannabis use and other illicit drug use.” (p. 43)

• “...the gateway hypothesis only addresses the order of drug use initiation, the gateway hypothesis does not specify any mechanistic connections between drug "stages" following exposure to marijuana and does not extend to the risks for addiction.” (p. 44)

• “Marijuana is not an immediate precursor of another controlled substance.” (p. 46)

• “The HHS reviewed the clinical studies evaluating the gateway hypothesis in marijuana and found them to be limited.” The DEA goes on to say, “The HHS cited several studies where marijuana use did not lead to other illicit drug use.” (p. 162)

• “Based on these studies among others, the HHS concluded that although many individuals with a drug abuse disorder may have used marijuana as one of their first illicit drugs, this does not mean that individuals initiated with marijuana inherently will go on to become regular users of other illicit drugs.” (p. 162)

Cannabis does not cause irreversible cognitive decline:

• “...cannabis-associated cognitive deficits are reversible and related to recent cannabis exposure, rather than irreversible and related to lifetime use.” (p. 20)

Cannabis does not cause psychosis:

• “At present, the available data do not suggest a causative link between marijuana use and the development of psychosis.” (p. 22)

Cannabis does not cause lung cancer:

• “…in a large study with 1,650 subjects, a positive association was not found between marijuana and lung cancer (Tashkin et al., 2006). This finding remained true, regardless of the extent of marijuana use, when controlling for tobacco use and other potential confounding variables.”  (p. 11)

• “Overall, new evidence suggests that the effects of marijuana smoking on respiratory function and carcinogenicity differ from those of tobacco smoking.” (p. 11)

• The DEA further notes the publication of recent review articles critically evaluating the association between marijuana and lung cancer. Most of the reviews agree that the association is weak or inconsistent (Huang et al., 2015; Zhang et al., 2015; Gates et al., 2014; Hall and Degenhardt, 2014).” (p.65)
  
  

Relevant supporting studies from “Denial of Petition to Initiate Proceedings to Reschedule Marijuana” (http://safeaccessnow.org/dea_denial)  

1. Pope, H. G., Gruber, A. J., Hudson, J. I., Huestis, M. A., & Yurgelun-Todd, D. (2001). Neuropsychological performance in long-term cannabis users. Archives of General Psychiatry, 58(10), 909–915.

2. Degenhardt, L., Chiu, W. T., Conway, K., Dierker, L., Glantz, M., Kalaydjian, A., et al. (2009). Does the “gateway” matter? Associations between the order of drug use initiation and the development of drug dependence in the National Comorbidity Study Replication. Psychological Medicine, 39(1), 157–167. http://doi.org/10.1017/S0033291708003425

3. Fergusson, D. M., Horwood, L. J., & Ridder, E. M. (2005). Tests of causal linkages between cannabis use and psychotic symptoms. Addiction, 100(3), 354–366.

4. Degenhardt, L., Hall, W., & Lynskey, M. (2003). Testing hypotheses about the relationship between cannabis use and psychosis. Drug Alcohol Depend, 71(1), 37–48.

5. Gong H. Jr, Tashkin DP, Simmons MS, Calvarese B, Shapiro BJ. Acute and subacute bronchial effects of oral cannabinoids. Clin Pharmacol Ther. 1984 Jan; 35(1):26–32.

6. Kandel, D. B., & Chen, K. (2000). Types of marijuana users by longitudinal course. J Stud Alcohol, 61(3), 367–378. http://doi.org/10.15288/jsa.2000.61.367

7. Lee, M. H. S., & Hancox, R. J. (2011). Effects of smoking cannabis on lung function. Expert Review of Respiratory Medicine, 5(4), 537–46– quiz 547. http://doi.org/10.1586/ers.11.40

8. Kuepper, R., van Os, J., Lieb, R., Wittchen, H. U., Hofler, M., & Henquet, C. (2011). Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study. BMJ, 342,, 738–738. http://doi.org/10.1136/bmj.d738

9. Minozzi, S., Davoli, M., Bargagli, A. M., Amato, L., Vecchi, S., & Perucci, C. A. (2010). An overview of systematic reviews on cannabis and psychosis: Discussing apparently conflicting results. Drug and Alcohol Review, 29(3), 304–317. http://doi.org/10.1111/j.1465-3362.2009.00132.x

10. Gong H. Jr, Tashkin DP, Simmons MS, Calvarese B, Shapiro BJ. Acute and subacute bronchial effects of oral cannabinoids. Clin Pharmacol Ther. 1984 Jan; 35(1):26–32.

11. Zhang ZF, Morgenstern H, Spitz MR, Tashkin DP, Yu GP, Marshall JR, Hsu TC, Schantz SP. Marijuana use and increased risk of squamous cell carcinoma of the head and neck. Cancer Epidemiol Biomarkers Prev. 1999 Dec;8(12): 1071–8.

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