The "gateway hypothesis" has been around for decades and is often used to rebut arguments for legalizing pot. But according to German Lopez at Vox, there is no solid evidence to support the idea that pot is somehow a "gateway" to harder drugs.
It's true that marijuana use correlates with harder drug use. But so does alcohol and tobacco use. There doesn't have to be a causal link between marijuana or alcohol and harder drugs to explain this; it could just be that the things that drive someone to marijuana or alcohol — boredom, depression, social circles — can just as easily drive them to other drugs. Perhaps the correlation is actually exposing those underlying factors, not some gateway effect.
So there is no good causal evidence for the gateway hypothesis. There is some weak correlational evidence, but it can be easily support with an entirely different idea — drug users tend to start with more accessible drugs — over the gateway hypothesis.
The analysis indicated that respondents who reported past-year marijuana use in their initial interview had 2.2 times higher odds than nonusers of meeting DSM-IV diagnostic criteria for prescription opioid use disorder by the follow-up. They also had 2.6 times greater odds of initiating prescription opioid misuse, defined as using a drug without a prescription, in higher doses, for longer periods, or for other reasons than prescribed.
But experts have argued as to whether the relationship between marijuana use and opioid addiction is causal or just correlated.