Joe Biden has said he would not legalise cannabis across the US because he feared it could be a gateway drug.

The Democratic presidential candidate said he needed “a lot more” evidence on whether the substance caused further drug use.

The US National Institute on Drug Abuse (NIDA) says most cannabis users do not go on to use harder substances.

"There has not been nearly enough evidence that has been acquired as to whether or not it is a gateway drug,” Mr Biden said during a town hall meeting in Las Vegas.

“It’s a debate. I want a lot more before I legalise it nationally. I want to make sure we know a lot more about the science behind it.”

Where is cannabis legal in the US? Show all 11 1 /11 Where is cannabis legal in the US? Where is cannabis legal in the US? Alaska Recreational cannabis use was made legal in Alaska in 2014 Getty Where is cannabis legal in the US? Washington Washington was among the first states to legalise cannabis in 2012 Getty Where is cannabis legal in the US? Michigan Recreational cannabis use was made legal in Michigan in 2018 Getty Where is cannabis legal in the US? Massachusetts Recreational cannabis use was made legal in Massachusetts in 2016 Getty Where is cannabis legal in the US? Colorado Colorado was among the first states to legalise cannabis in 2012 Getty Where is cannabis legal in the US? Vermont Since January 2018 it has been legal to posses up to one ounce and two cannabis plants in Vermont. Commercial sales are not legal Getty Where is cannabis legal in the US? California Recreational cannabis use was made legal in California in 2018 Getty Where is cannabis legal in the US? Illinois Recreational cannabis use was made legal in Illinois in 2019 Getty Images Where is cannabis legal in the US? Oregon It was made legal in Oregon to possess up to one ounce of cannabis in 2014 Getty Where is cannabis legal in the US? Maine Recreational cannabis use was made legal in Maine in 2016 Getty Where is cannabis legal in the US? Nevada Recreational cannabis use was made legal in Nevada in 2016 Getty

While some research points to cannabis acting as a "gateway" to harder substances, other research has found alcohol and nicotine may be more likely to lead to further drug use.

One study found that even though cannabis did often precede people taking other drugs, the reason why subjects used cannabis in the first place - such as "boredom" - was a larger risk factor than the cannabis itself.

Mr Biden said he supported individual states deciding their own recreational cannabis laws.

He said: “I don’t know enough to know whether it is [a gateway drug] or not, although I’ve done a great deal of work on the drugs side of the issue.”

The former vice president under Barack Obama was asked during the meeting whether his stance towards recreational cannabis has changed. He replied: “No, it has not.”

Mr Biden has a long history of supporting anti-drug legislation and has been criticised for his role during the "war on drugs" of the 1990s.

While chair of the Senate Judiciary Committee, he helped write a “tough on crime” bill which led to harsh sentences for crack cocaine use and has been accused as disproportionately affecting black Americans.

However, during his talk in Las Vegas, he said he did not believe people should be sent to prison for cannabis use and that any existing convictions should be expunged.

He also said he supported medical use of the drug.