A popular and widely-held argument against the legalization of marijuana is that usage of the drug makes it more likely to use harder drugs. This is called the gateway theory because, in this view, the use of marijuana serves as a "gateway" into the heavier stuff. 

The gateway drug theory is, naturally, a controversial one. Evidence surrounding the nature of the gateway drug theory is mixed, with evidence seemingly both supporting and negating the theory. 

There does seem to be a link between marijuana use and use of other drugs. In one national study, two-thirds of marijuana users reported suffering from another addiction issue. In the study, marijuana users were 10 times as likely to have developed a new drug disorder than non-marijuana users. 

Additionally, a person who smokes marijuana is significantly more likely to begin using cocaine. There does appear to be a compelling link between marijuana usage and usage of other, harder, drugs. 

Marijuana use appears to create a greater sensitivity for other drugs, like heroin and cocaine. In a study, lab rats who were exposed to THC, a component in marijuana showed greater sensitivity towards other drugs, which suggests that use of one mind-altering substance promotes greater sensitivity towards other similar substances. 

However, establishing a causal link between marijuana usage and usage of other drugs is more difficult to prove. There are several additional theories that are consistently pushed as explanations for why marijuana users are more likely to turn towards harder drugs. 

There is a social aspect that argues that because marijuana is so common, it is more likely to have been exposed to marijuana before other drugs. Therefore, in this theory, marijuana does not cause the use of other drugs, but simply precedes it. 

The illegal nature of marijuana might also make it more likely for users to come in contact with other illegal substances, thus increasing their chances of trying and becoming addicted to other substances. This theory is supported by some evidence from the Netherlands, which has a regulated marijuana market. In the Netherlands, the usage of marijuana is less than in the United States.

Experts contribute the lower usage of marijuana in the Netherlands to the fact that its legality in the Netherlands separate the market for marijuana from the market for other drugs, making it less likely for marijuana users to come into contact with those harder markets.

As marijuana legalization becomes more accepted throughout the United States, and as states continue to debate the legalization of marijuana, it is important to continue to study if there is a legitimate linkage between marijuana use and use of harder drugs. The evidence surrounding that link is still mixed.