who reported using crack cocaine or heroin daily.³³ However, only a tiny fraction of the adult population—an estimated one to three per 1,000 people—uses crack or heroin this often. While most of the people interviewed for the study said they had used marijuana before moving on to harder drugs, that trend is not necessarily true among marijuana users in general.

Another drawback of many studies on the gateway theory is that they rely on measurements of drug use rather than drug dependence. Thus, these studies can only demonstrate that, when compared with people who have never used the drug, marijuana users are more likely to try other drugs as well; such studies do not prove that marijuana users tend to become dependent, or even frequent, users of harder drugs. The real value of this type of research is that it can reveal factors that predict whether a person will progress from using a given illegal drug to a harder one.

Marijuana is a gateway drug in the sense that its use typically precedes rather than follows initiation into other illicit substances. On the other hand, marijuana use per se does not appear to be a gateway to the extent that it is a cause, or even a significant predictor, of hard drug abuse. Instead, the most consistent predictors of hard drug abuse appear to be intense marijuana use, psychiatric disorders, and a family history of psychological problems or alcoholism.

It is also important to recognize that research on drug progression has focused exclusively on recreational use. It does not follow that if marijuana were available by prescription for medical use the pattern of subsequent drug progression among medical users would be the same as for recreational users. In fact, a study of nonmedical use of psychoactive prescription drugs—including tranquilizers, antidepressants, and opiate painkillers—failed to find that a clear or consistent sequence of drug use following the abuse of these medications. At present, data on drug use neither support nor refute the assertion that legalizing marijuana for medical purposes would prompt increased drug abuse among medical marijuana users.

A related but distinct concern is whether the use of marijuana for medical purposes would encourage drug use throughout society in general. Unless and until marijuana is approved for medical treatment, we can only speculate about the answer to this