Gateway drug theory
The gateway drug theory (also called gateway theory, gateway hypothesis and gateway effect) states that the use of less deleterious drugs can lead to a future risk of using more dangerous hard drugs or crime. It is often attributed to the earlier use of one of several licit substances, including tobacco or alcohol, as well as cannabis.
The reverse gateway theory posits that earlier regular cannabis use predicts later tobacco initiation and/or nicotine dependence in those who did not use tobacco before.
The hypothesis is that the use of soft drugs like cannabis leads to the use of harder drugs via a sequence of stages. This is based on the observation that many consumers who use cocaine or heroin have previously used cannabis, and most have used alcohol or tobacco; the hypothesis is that progression continues from there to other drugs like cocaine or heroin. Some research supports that cannabis use predicts a significantly higher risk for subsequent use of "harder" illicit drugs, while other research does not. Some research even has found alcohol represented the "gateway" drug, leading to the use of tobacco, marijuana, and other illicit substances.
While some research shows that many hard drug users used cannabis or alcohol before moving on to the harder substances, other research shows that some serious drug abusers have not used alcohol or cannabis first. The latter is evident in Japan, where the overwhelming majority of users of illicit drugs do not use cannabis first. The risk factor for using drugs in cannabis users may be higher because few people try hard drugs prior to trying cannabis, not because cannabis users increasingly try hard drugs such as certain substituted amphetamines (e.g., methamphetamine). For example, cannabis is typically available at a significantly earlier age than other illicit drugs.
Jacob Sullum analyzed the "gateway" theory in a 2003 Reason magazine article, noting that the theory's "...durability is largely due to its ambiguity: Because it's rarely clear what people mean when they say that pot smoking leads to the use of "harder" drugs, the claim is difficult to disprove.":
- "Notice that none of these interpretations involves a specific pharmacological effect of the sort drug warriors seem to have in mind when they suggest that pot smoking primes the brain for cocaine or heroin. As a National Academy of Sciences panel observed in a 1999 report, 'There is no evidence that marijuana serves as a stepping stone on the basis of its particular drug effect.' Last year the Canadian Senate's Special Committee on Illegal Drugs likewise concluded that 'cannabis itself is not a cause of other drug use. In this sense, we reject the gateway theory.'"
The National Institute on Drug Abuse is not convinced that cannabis is a gateway drug. They have noted that most cannabis users do not go on to use "harder" substances. When compared with that of alcohol or tobacco, there is nothing particularly different about any pharmacological effect of cannabis that would make it more likely to cause a person to use more harmful substances. Alternatively, they suggest that some individuals may be more prone to using drugs and that these people are more likely to start with readily available substances such as cannabis, tobacco, and alcohol. 
Alcohol and tobacco
Alcohol tends to precede cannabis use, and it is rare for those who use hard drugs to not have used alcohol or tobacco first; the 2005 National Survey of Drug Use and Health (NSDUH) in the United States found that, compared with lifetime nondrinkers, adults who have consumed alcohol were statistically much more likely to currently use illicit drugs and/or abuse prescription drugs in the past year. Effects were strongest for cocaine (26 times more likely), cannabis (14 times more likely), and psychedelics (13 times more likely). In addition, lifetime drinkers were also six times more likely to use or be dependent on illicit drugs than lifetime nondrinkers.
According to the NIDA, "People who abuse drugs are also likely to be cigarette smokers. More than two-thirds of drug abusers are regular tobacco smokers, a rate more than triple that of the rest of the population."
Alternative explanations for the correlation between the use of soft drugs (e.g., marijuana) and the use of hard drugs (e.g., cocaine, heroin) include, but are not limited to:
- The Common Liability to Addiction theory (CLA) states that some individuals are, for various reasons, willing to try multiple recreational substances. The "gateway" drugs are merely those that are (usually) available at an earlier age than the harder drugs. Researchers have noted in an extensive review, Vanyukov et al., that it is dangerous to present the sequence of events described in gateway "theory" in causative terms as this hinders both research and intervention. 
- Teenagers' trust of adults erodes when authority-figures exaggerate or make up the dangers of the "gateway" drugs, leading teenagers to regard all anti-drug messages as nonsense.
- The peer environments in which "gateway" drugs are used can sometimes overlap with the ones in which harder drugs are used, especially in societies that prohibit the substances or impose very high age-limits.
- Vanyukov MM, Tarter RE, Kirillova GP et al. (June 2012). "Common liability to addiction and "gateway hypothesis": theoretical, empirical and evolutionary perspective". Drug Alcohol Depend (Review). 123 Suppl 1: S3–17. doi:10.1016/j.drugalcdep.2011.12.018. PMC 3600369. PMID 22261179.
- Pudney, Stephen (December 2002). "The road to ruin? Sequences of initiation into drug use and offending by young people in Britain" (PDF). Home Office Research Study 253. (London: Home Office Research, Development and Statistics Directorate). ISBN 1-84082-928-1. ISSN 0072-6435. Retrieved 2009-04-04.
- Peters EN, Budney AJ, Carroll KM (August 2012). "Clinical correlates of co-occurring cannabis and tobacco use: a systematic review". Addiction (Review) 107 (8): 1404–17. doi:10.1111/j.1360-0443.2012.03843.x. PMC 3377777. PMID 22340422.
- Peters EN, Budney AJ, Carroll KM (August 2012). "Clinical correlates of co-occurring cannabis and tobacco use: a systematic review". Addiction 107 (8): 1404–17. doi:10.1111/j.1360-0443.2012.03843.x. PMC 3377777. PMID 22340422.
- Sullum, Jacob (24 Jan 2003), Marijuana as a "gateway" drug, Reason, retrieved 2014-04-01
- "Is marijuana a gateway drug?". drugabuse.gov. National Institute of Drug Abuse.
- Illicit Drug Use among Lifetime Nondrinkers and Lifetime Alcohol Users, NSDUH, 2005
- The National Institute on Drug Abuse (NIDA), part of the NIH, a component of the U.S. Department of Health and Human Services. – Nicotine Craving and Heavy Smoking May Contribute to Increased Use of Cocaine and Heroin – Patrick Zickler, NIDA NOTES Staff Writer. Retrieved October, 2006.
- Brecher, Edward M. (1972). "Heroin on the youth drug scene - and in Vietnam". Licit and illicit drugs; the Consumers Union report on narcotics, stimulants, depressants, inhalants, hallucinogens, and marijuana - including caffeine, nicotine, and alcohol. Boston: Little, Brown. ISBN 0-316-10717-4.
- Ginzler JA, Cochran BN, Domenech-Rodríguez M, Cauce AM, Whitbeck LB (2003). "Sequential progression of substance use among homeless youth: an empirical investigation of the gateway theory". Subst Use Misuse (Review) 38 (3-6): 725–58. doi:10.1081/ja-120017391. PMID 12747403.
- Kenkel D, Mathios AD, Pacula RL (January 2001). "Economics of youth drug use, addiction and gateway effects". Addiction (Review) 96 (1): 151–64. doi:10.1080/09652140020017021. PMID 11177526.
- The classification of cannabis under the Misuse of Drugs Act 1971 (Section 4.6 "Does cannabis use lead on to other drug use?")
- How did the marijuana gateway myth get started? Schaffer Library of Drug Policy.