What is a gateway drug?
First, it is reasonable to think that early marijuana use can provide a gateway to trying harder drugs. After all, there are almost no heroin users who didn't start their illegal drug use with marijuana, and marijuana smokers are 104 times more likely to use cocaine than those haven't tried weed. Furthermore, studies in rodents suggest that early marijuana use leads to an increased vulnerability for drug abuse and addiction to other substances later in life. This is also consistent with animal experiments showing THC’s ability to "prime" the brain for enhanced responses to other drugs. For example, rats previously administered THC show heightened behavioral response not only when further exposed to THC but also when exposed to other drugs such as morphine—a phenomenon called cross-sensitization. These findings are consistent with the idea of marijuana as a "gateway drug."
However, most people who use marijuana do not go on to use other, "harder" substances. Also, cross-sensitization is not unique to marijuana. Alcohol and nicotine also prime the brain for a heightened response to other drugs and are, like marijuana, also typically used before a person progresses to other, more harmful substances. So if we call marijuana a "gateway" drug, shouldn't we also include nicotine and alcohol in this category? It is important to note that other factors besides biological mechanisms, such as a person’s social environment, are also critical in a person’s risk for drug use.
An alternative to the gateway-drug hypothesis is that people who are more vulnerable to drug-taking are simply more likely to start with readily available substances like marijuana, tobacco, or alcohol, and their subsequent social interactions with other drug users increases their chances of trying other drugs.
This begs the question, is marijuana the main culprit, acting as a gateway to harder, illegal drugs? Or is is the environment and social factors, which really lead a person down a path of hard drug use? While there are certainly correlations between the early use of marijuana and later use of hard drugs, these are only correlations. Most people do not start with the hard stuff, such as cocaine or heroin. However, using marijuana before moving on to the harder drug shouldn't be viewed as a cause and effect scenario. Odds are these people may have used nicotine and/or alcohol before the hard drugs also.
Instead of labeling marijuana as a gateway drug, which is essentially a way to place blame on the substance, what should be examined are the underlying psychological or social issues, which lead to a progression of harder drug use. The gateway argument is of the chicken before the egg variety, and specious, at best. It is of course important to understand how someone has arrived to an addiction - but to fully understand this and work towards recovery - it's not as important to know which drug leads to another.
It's most important to know the steps to recovery.