Anti-cannabis crusaders have called it a "gateway drug" for decades.

But a new study by two Canadian universities suggests that's not only wrong, but ignores the safety of cannabis compared with other illicit substances, including alcohol.

The study looked at 473 medical cannabis users, of which 87 percent reported substituting marijuana for alcohol, illicit drugs or prescription drugs.

"While cannabis is not benign, most research suggests that it's safer and less addictive than many substances, particularly prescription opiates," wrote the study's lead author, Philippe Lucas.

Marijuana could, Lucas adds, actually be an "exit drug" as people quit harder stuff in favour of lighting up.

The notion of marijuana (or anything) as a "gateway drug" is notoriously difficult to prove or disprove. As Reason's Jacob Sullum points out in a 2003 article, anti-drug warriors rarely cite the specific, pharmacological effect of cannabis when they claim pot smoking primes the brain for cocaine or heroin.

Sullum backs up this claim by referencing reports as far back as 1999, when the U.S. National Academy of Sciences panel said, "There is no evidence that marijuana serves as a stepping stone on the basis of its particular drug effect."

In 2002, writes Sullum, the Canadian Senate's Special Committee on Illegal Drugs also concluded that, "cannabis itself is not a cause of other drug use. In this sense, we reject the gateway theory."

According to Lucas, who also works for medical cannabis cultivation firm Tilray, the data on pot's safety vis-a-vis other drugs should pique the interest of government.

"Research suggesting that cannabis substitution could reduce harms and lessen the public health and safety impact of alcohol and other drugs has significant policy implications."

h/t CBC, Reason