Let us examine the literature. The risk of progressing from marijuana to cocaine or heroin is now well documented. Marijuana use among adolescents appears to be a symptom of problem behavior, not a cause. Scientific theories are scientific because they are the basis for consistently successful predictions. Gravity is a prediction that when apples leave a tree branch, they fall.
Gravity is a scientific theory because things fall, largely without exception.
The Office of Technology Assessment does not say that the gateway drug theory is unscientific, but they reach a similar conclusion. Furthermore, it is important to keep in mind that although a clear development sequence in drug involvement has been identified, use of a drug at a particular stage does not invariably lead to the use of other drugs higher up in the sequence. They conclude that teenagers are especially at risk to drug abuse if they begin to use marijuana. The dangers have less to do with marijuana itself than with use of marijuana by adolescents. Yamaguchi and Kandel's findings deserve close consideration, and must not be removed from their original context.