Marijuana is a gateway drug
Those who regularly smoke marijuana are 60 times more likely to try other illegal drugs than those who have never tried it. By legalizing marijuana, we legitimize the first step in a pipeline that leads users to harder, far more dangerous drugs. People who use marijuana use more drugs (legal and illegal) than people that do not use marijuana. Also, people that are addicted to marijuana are three times more likely to be addicted to heroin. Legalizing marijuana makes it more available and acceptable to use. This will result in an increase in usage which can lead to an increase in heroin use. To address the opioid crisis, we need to discourage drug usage, not legalize it. Marijuana also increases vulnerability for addiction. Experiments on animals have shown that marijuana use decreases the reactivity of the brain’s dopamine reward center and primes the brain for an enhanced reaction to other drugs such as morphine. These reactions both increase the vulnerability for addiction. Therefore, not only does marijuana use lead to other harder drug use, but it makes those users more susceptible to addiction. Marijuana can lead people down a dangerous and addictive path toward harder and more dangerous drugs, such as heroin.
Most people who use marijuana do not go on to become addicted to harder drugs. Even for those that do go on to use harder drugs, it cannot be determined that marijuana use is a precipitating factor. The correlation could be showing that the things that drive someone to use marijuana or alcohol (boredom, depression, or their social circle) also drive people to harder drug use. Even if marijuana is a gateway drug, there is also research that both alcohol and tobacco increase the likelihood of using harder drugs. It doesn’t make sense to use the gateway argument as a rationale for marijuana to be illegal when alcohol and cigarettes are both gateway drugs and are legal. In fact, most users of harder drugs begin with alcohol and cigarettes before moving to marijuana and then additional drugs. This shows that marijuana isn’t the first or most predominant gateway drug.
[P1] Marijuana is a gateway drug. [P2] Marijuana increases vulnerability to addiction. [P3] Legalizing marijuana would lead more users to marijuana and therefore more people would move on to become addicted to more dangerous drugs.
Rejecting the premises
[Rejecting P1] Both alcohol and tobacco could also be characterized as gateway drugs. This is not a rationale for them to be illegal. [Rejecting P3] We do not know that more people would use marijuana and the gateway effect may only show correlation, not causation.