Legalising marijuana doesn't encourage people to start smoking, new research suggests.

Researchers in the US looked at data from between 1984 and 2015, comparing rates of marijuana use and when laws changed across different states of the US.

Twenty-nine states, as well as Washington DC, have medicinal cannabis schemes, and eight states have legalised the hallucinogenic drug for adults.

They found rather than legalisation or decriminalisation preceding increased rates of marijuana use, it was usually the other way around.

"Medical and recreational marijuana policies did not have any significant association with increased marijuana use," the study, published in journal Addiction, found.

"Marijuana policy liberalisation over the past 20 years has certainly been associated with increased marijuana use; however, policy changes appear to have occurred in response to changing attitudes within states."

That is, states are changing their laws because more people either want to smoke, or don't mind if others do.

A separate look at the effect of marijuana legalisation and its effect on usage rates also came out this week, this one put together by researchers at Harvard University and Western Carolina University.

It concludes "marijuana liberalisations have had minimal impact" on smoking rates, and may even be contributing to lower rates of cocaine and heroin use.

Newshub.