marijuana is a gateway drug - if you believe in prohibition...,

NYTimes |  It should come as no surprise that the vast majority of heroin users have used marijuana (and many other drugs) not only long before they used heroin but while they are using heroin. Like nearly all people with substance abuse problems, most heroin users initiated their drug use early in their teens, usually beginning with alcohol and marijuana. There is ample evidence that early initiation of drug use primes the brain for enhanced later responses to other drugs. These facts underscore the need for effective prevention to reduce adolescent use of alcohol, tobacco and marijuana in order to turn back the heroin and opioid epidemic and to reduce burdens addiction in this country. 

Marijuana use is positively correlated with alcohol use and cigarette use, as well as illegal drugs like cocaine and methamphetamine. This does not mean that everyone who uses marijuana will transition to using heroin or other drugs, but it does mean that people who use marijuana also consume more, not less, legal and illegal drugs than do people who do not use marijuana.

People who are addicted to marijuana are three times more likely to be addicted to heroin.

The legalization of marijuana increases availability of the drug and acceptability of its use. This is bad for public health and safety not only because marijuana use increases the risk of heroin use.

A better drug policy is one that actively discourages marijuana use as well as other recreational drug use, especially for youth.