The Gateway Hypothesis states that cannabis use increases the probability of trying "harder" drugs. The hypothesis has been hotly debated as it is regarded by some as the primary rationale for the United States prohibition on cannabis use.[30][31] A Pew Research Center poll found that political opposition to marijuana use was significantly associated with concerns about health effects and whether legalization would increase marijuana use by children.[32]
Some studies state that while there is no proof for the gateway hypothesis,[33] young cannabis users should still be considered as a risk group for intervention programs.[34] Other findings indicate that hard drug users are likely to be poly-drug users, and that interventions must address the use of multiple drugs instead of a single hard drug.[35] Almost two-thirds of the poly drug users in the "2009/10 Scottish Crime and Justice Survey" used cannabis.[36]
The gateway effect may appear due to social factors involved in using any illegal drug. Because of the illegal status of cannabis, its consumers are likely to find themselves in situations allowing them to acquaint with individuals using or selling other illegal drugs.[37][38] Utilizing this argument some studies have shown that alcohol and tobacco may additionally be regarded as gateway drugs;[39] however, a more parsimonious explanation could be that cannabis is simply more readily available (and at an earlier age) than illegal hard drugs. In turn alcohol and tobacco are easier to obtain at an earlier point than is cannabis (though the reverse may be true in some areas), thus leading to the "gateway sequence" in those individuals since they are most likely to experiment with any drug offered.[30]
An alternative to the gateway hypothesis is the Common Liability to Addiction theory (CLA). It 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.